diff --git a/egs/librispeech/ASR/zipformer/scaling.py b/egs/librispeech/ASR/zipformer/scaling.py
index 7c98ef045..a8b63184f 100644
--- a/egs/librispeech/ASR/zipformer/scaling.py
+++ b/egs/librispeech/ASR/zipformer/scaling.py
@@ -1602,6 +1602,61 @@ def convert_num_channels(x: Tensor, num_channels: int) -> Tensor:
         return torch.cat((x, zeros), dim=-1)
 
 
+def unfold(
+    x: Tensor, x_pad: int, num_blocks: int, kernel: int, stride: int, padding: int
+) -> Tensor:
+    """
+    Args:
+        x: input of shape (seq_len, batch_size, channel)
+    Returns:
+        blocks: (kernel, batch_size * num_blocks, channel)
+    """
+    seq_len, batch_size, channel = x.size()
+    x = x.permute(1, 2, 0)  # (batch_size, channel, seq_len)
+
+    x = nn.functional.pad(x, pad=(0, x_pad), value=0.0)
+
+    blocks = nn.functional.unfold(
+        x.unsqueeze(-1),
+        kernel_size=(kernel, 1),
+        padding=(padding, 0),
+        stride=(stride, 1),
+    )  # (B, C * kernel, num_blocks)
+    blocks = blocks.reshape(batch_size, channel, kernel, num_blocks)
+    blocks = blocks.permute(2, 0, 3, 1)
+    blocks = blocks.reshape(kernel, batch_size * num_blocks, channel)
+
+    return blocks
+
+
+def fold(
+    blocks: Tensor, seq_len: int, x_pad: int, num_blocks: int, kernel: int, stride: int, padding: int
+) -> Tensor:
+    """
+    Args:
+        blocks: (kernel, batch_size * num_blocks, channel)
+    Returns:
+        x: (seq_len, batch_size, channel)
+    """
+    batch_size = blocks.size(1) // num_blocks
+    channel = blocks.size(2)
+
+    blocks = blocks.reshape(kernel, batch_size, num_blocks, channel)
+    blocks = blocks.permute(1, 3, 0, 2).reshape(batch_size, channel * kernel, num_blocks)
+
+    x = nn.functional.fold(
+        blocks,
+        output_size=(seq_len + x_pad, 1),
+        kernel_size=(kernel, 1),
+        padding=(padding, 0),
+        stride=(stride, 1),
+    )
+    x = x.squeeze(-1).permute(2, 0, 1)
+    x = x[:seq_len]  # (seq_len, batch_size, channel)
+
+    return x
+
+
 def _test_whiten():
     for proportion in [0.1, 0.5, 10.0]:
         logging.info(f"_test_whiten(): proportion = {proportion}")
diff --git a/egs/librispeech/ASR/zipformer/train.py b/egs/librispeech/ASR/zipformer/train.py
index bc3e9c1ba..9b2dd8a95 100755
--- a/egs/librispeech/ASR/zipformer/train.py
+++ b/egs/librispeech/ASR/zipformer/train.py
@@ -187,6 +187,13 @@ def add_model_arguments(parser: argparse.ArgumentParser):
         help="Positional-encoding embedding dimension",
     )
 
+    parser.add_argument(
+        "--max-block-size",
+        type=str,
+        default="512",
+        help="Max block size used in block-wise attention; a single int or comma-separated list",
+    )
+
     parser.add_argument(
         "--encoder-unmasked-dim",
         type=str,
@@ -574,6 +581,7 @@ def get_encoder_model(params: AttributeDict) -> nn.Module:
         num_heads=_to_int_tuple(params.num_heads),
         feedforward_dim=_to_int_tuple(params.feedforward_dim),
         cnn_module_kernel=_to_int_tuple(params.cnn_module_kernel),
+        max_block_size=_to_int_tuple(params.max_block_size),
         dropout=ScheduledFloat((0.0, 0.3), (20000.0, 0.1)),
         warmup_batches=4000.0,
         causal=params.causal,
diff --git a/egs/librispeech/ASR/zipformer/zipformer.py b/egs/librispeech/ASR/zipformer/zipformer.py
index b39af02b8..e305c10e7 100644
--- a/egs/librispeech/ASR/zipformer/zipformer.py
+++ b/egs/librispeech/ASR/zipformer/zipformer.py
@@ -39,6 +39,8 @@ from scaling import (
     FloatLike,
     limit_param_value,
     convert_num_channels,
+    fold,
+    unfold,
 )
 from torch import Tensor, nn
 
@@ -105,6 +107,8 @@ class Zipformer2(EncoderInterface):
             feedforward_dim: Union[int, Tuple[int]] = 1536,
             cnn_module_kernel: Union[int, Tuple[int]] = 31,
             pos_dim: int = 192,
+            max_block_size: Union[int, Tuple[int]] = 512,
+            block_pad: int = 16,
             dropout: FloatLike = None,  # see code below for default
             warmup_batches: float = 4000.0,
             causal: bool = False,
@@ -140,6 +144,7 @@ class Zipformer2(EncoderInterface):
         self.num_heads = num_heads = _to_tuple(num_heads)
         feedforward_dim = _to_tuple(feedforward_dim)
         self.cnn_module_kernel = cnn_module_kernel = _to_tuple(cnn_module_kernel)
+        self.max_block_size = max_block_size = _to_tuple(max_block_size)
 
         self.causal = causal
         self.chunk_size = chunk_size
@@ -153,6 +158,7 @@ class Zipformer2(EncoderInterface):
 
         num_encoders = len(downsampling_factor)
         for i in range(num_encoders):
+            ds = downsampling_factor[i]
 
             encoder_layer = Zipformer2EncoderLayer(
                 embed_dim=encoder_dim[i],
@@ -173,13 +179,15 @@ class Zipformer2(EncoderInterface):
                 encoder_layer,
                 num_encoder_layers[i],
                 pos_dim=pos_dim,
+                max_block_size=max_block_size[i],
+                block_pad=block_pad,
                 dropout=dropout,
                 warmup_begin=warmup_batches * (i + 1) / (num_encoders + 1),
                 warmup_end=warmup_batches * (i + 2) / (num_encoders + 1),
                 final_layerdrop_rate=0.035 * (downsampling_factor[i] ** 0.5),
             )
 
-            if downsampling_factor[i] != 1:
+            if ds != 1:
                 encoder = DownsampledZipformer2Encoder(
                     encoder,
                     dim=encoder_dim[i],
@@ -674,6 +682,8 @@ class Zipformer2EncoderLayer(nn.Module):
         pos_emb: Tensor,
         chunk_size: int = -1,
         attn_mask: Optional[Tensor] = None,
+        attn_offsets: Optional[Tensor] = None,
+        all_pad_mask: Optional[Tensor] = None,
         src_key_padding_mask: Optional[Tensor] = None,
     ) -> Tensor:
         """
@@ -681,6 +691,8 @@ class Zipformer2EncoderLayer(nn.Module):
         Args:
             src: the sequence to the encoder (required): shape (seq_len, batch_size, embedding_dim).
          pos_emb: (1, 2*seq_len-1, pos_emb_dim) or (batch_size, 2*seq_len-1, pos_emb_dim)
+        block_size: size of block
+        block_pad: pad size at each side of block
          chunk_size: the number of frames per chunk, of >= 0; if -1, no chunking.
        feature_mask: something that broadcasts with src, that we'll multiply `src`
               by at every layer: if a Tensor, likely of shape (seq_len, batch_size, embedding_dim)
@@ -706,7 +718,8 @@ class Zipformer2EncoderLayer(nn.Module):
             src,
             pos_emb=pos_emb,
             attn_mask=attn_mask,
-            key_padding_mask=src_key_padding_mask,
+            attn_offsets=attn_offsets,
+            all_pad_mask=all_pad_mask,
         )
 
         src = src + self.feed_forward1(src)
@@ -725,7 +738,8 @@ class Zipformer2EncoderLayer(nn.Module):
             selected_attn_weights = (selected_attn_weights > 0.0).to(selected_attn_weights.dtype)
             selected_attn_weights = selected_attn_weights * (1.0 / selected_attn_weights.sum(dim=-1, keepdim=True))
 
-        na = self.balancer_na(self.nonlin_attention(src, selected_attn_weights))
+        na = self.nonlin_attention(src, selected_attn_weights)
+        na = self.balancer_na(na)
 
         src = src + (na if self_attn_dropout_mask is None else na * self_attn_dropout_mask)
 
@@ -917,9 +931,11 @@ class Zipformer2Encoder(nn.Module):
             encoder_layer: nn.Module,
             num_layers: int,
             pos_dim: int,
+            max_block_size: int,
             dropout: float,
             warmup_begin: float,
             warmup_end: float,
+            block_pad: int = 16,
             initial_layerdrop_rate: float = 0.5,
             final_layerdrop_rate: float = 0.05,
     ) -> None:
@@ -931,6 +947,8 @@ class Zipformer2Encoder(nn.Module):
             [copy.deepcopy(encoder_layer) for i in range(num_layers)]
         )
         self.num_layers = num_layers
+        self.max_block_size = max_block_size
+        self.block_pad = block_pad
 
         assert 0 <= warmup_begin <= warmup_end
 
@@ -957,7 +975,7 @@ class Zipformer2Encoder(nn.Module):
             src: the sequence to the encoder (required): shape (seq_len, batch_size, embedding_dim).
             chunk_size: the number of frames per chunk, of >= 0; if -1, no chunking.
             feature_mask: something that broadcasts with src, that we'll multiply `src`
-               by at every layer: if a Tensor, likely of shape (seq_len, batch_size, embedding_dim)
+               by at every layer: if a Tensor, likely of shape (1, batch_size, embedding_dim)
             attn_mask: the attention mask, of shape (batch_size, seq_len, seq_len) or (seq_len, seq_len),
                  interpreted as (batch_size, tgt_seq_len, src_seq_len) or (tgt_seq_len, src_seq_len).
                  True means masked position. May be None.
@@ -966,6 +984,70 @@ class Zipformer2Encoder(nn.Module):
 
         Returns: a Tensor with the same shape as src.
         """
+        seq_len, batch_size, channel = src.size()
+        max_block_size = self.max_block_size
+        block_pad = self.block_pad
+
+        if seq_len > max_block_size:
+            # divide into blocks with overlaps
+            num_blocks = math.ceil(seq_len / max_block_size)
+            block_size = math.ceil(seq_len / num_blocks)
+            pad_len = num_blocks * block_size - seq_len
+            kernel_size = block_size + 2 * block_pad
+            if random.random() < 0.2 or __name__ == "__main__":
+                logging.info(f"seq_len={seq_len}, block_size={block_size}, pad_len={pad_len}")
+
+            # (block_size + 2 * block_pad, batch_size * num_blocks, channel)
+            src = unfold(
+                src, pad_len, num_blocks,
+                kernel=kernel_size, stride=block_size, padding=block_pad
+            )
+
+            # Used to mask out the padding positions
+            attn_offsets = torch.ones(batch_size, seq_len, device=src.device)
+
+            if src_key_padding_mask is not None:
+                assert src_key_padding_mask.shape == (batch_size, seq_len), src_key_padding_mask.shape
+                attn_offsets = attn_offsets.masked_fill(src_key_padding_mask, 0.0)  # 0 at padding positions
+            # (seq_len, batch, 1)
+            attn_offsets = attn_offsets.transpose(0, 1).unsqueeze(-1)
+            # (kernel_size, new_batch_size)
+            attn_offsets = unfold(
+                attn_offsets, pad_len, num_blocks,
+                kernel=kernel_size, stride=block_size, padding=block_pad,
+            ).squeeze(-1)
+
+            # Used for the blocks are all padding
+            all_pad_mask = (attn_offsets.sum(dim=0, keepdim=True) == 0)   # (1, new_batch_size)
+            all_pad_mask = all_pad_mask.unsqueeze(-1).unsqueeze(-1)  # (1, new_batch_size, 1, 1)
+
+            # (new_batch_size, kernel_size)
+            src_key_padding_mask = (attn_offsets == 0).transpose(0, 1)
+
+            attn_offsets = 1 - attn_offsets  # 1 at padding positions
+            attn_offsets[attn_offsets != 0] = -1000
+
+            # (1, new_batch_size, 1, kernel)
+            attn_offsets = attn_offsets.transpose(0, 1).unsqueeze(1).unsqueeze(0)
+
+            # feature_mask: (1, batch_size, channel)
+            if isinstance(feature_mask, Tensor):
+                feature_mask = feature_mask.unsqueeze(2).expand(-1, -1, num_blocks, -1)
+                # now (kernel_size, batch_size, num_blocks, channel)
+                feature_mask = feature_mask.reshape(1, batch_size * num_blocks, channel)
+        else:
+            block_size = 0
+
+            # Used to mask out the padding positions
+            attn_offsets = torch.zeros(batch_size, seq_len, device=src.device)
+            if src_key_padding_mask is not None:
+                assert src_key_padding_mask.shape == (batch_size, seq_len), src_key_padding_mask.shape
+                attn_offsets = attn_offsets.masked_fill(src_key_padding_mask, -1000)  # 0 at padding positions
+                # (1, batch_size, 1, seq_len)
+                attn_offsets = attn_offsets.unsqueeze(1).unsqueeze(0)
+
+            all_pad_mask = None
+
         pos_emb = self.encoder_pos(src)
         output = src
 
@@ -978,12 +1060,29 @@ class Zipformer2Encoder(nn.Module):
                 pos_emb,
                 chunk_size=chunk_size,
                 attn_mask=attn_mask,
+                attn_offsets=attn_offsets,
+                all_pad_mask=all_pad_mask,
                 src_key_padding_mask=src_key_padding_mask,
             )
 
             if not torch.jit.is_scripting() and not torch.jit.is_tracing():
                 output = output * feature_mask
 
+        if seq_len > max_block_size:
+            # overlap-and-add
+            output = fold(
+                output, seq_len, pad_len, num_blocks,
+                kernel=kernel_size, stride=block_size, padding=block_pad
+            )  # (seq_len, batch_size, channel)
+            mask = torch.ones(
+                kernel_size, batch_size * num_blocks, 1, device=src.device,
+            )
+            mask = fold(
+                mask, seq_len, pad_len, num_blocks,
+                kernel=kernel_size, stride=block_size, padding=block_pad
+            )  # (seq_len, batch_size, 1)
+            output = output / mask
+
         return output
 
     def streaming_forward(
@@ -1314,9 +1413,9 @@ class CompactRelPositionalEncoding(torch.nn.Module):
         self.length_factor = length_factor
         self.extend_pe(torch.tensor(0.0).expand(max_len))
 
-    def extend_pe(self, x: Tensor, left_context_len: int = 0) -> None:
+    def extend_pe(self, x: Tensor) -> None:
         """Reset the positional encodings."""
-        T = x.size(0) + left_context_len
+        T = x.size(0)
 
         if self.pe is not None:
             # self.pe contains both positive and negative parts
@@ -1361,25 +1460,24 @@ class CompactRelPositionalEncoding(torch.nn.Module):
 
         self.pe = pe.to(dtype=x.dtype)
 
-    def forward(self, x: Tensor, left_context_len: int = 0) -> Tensor:
+    def forward(self, x: Tensor, rel_pos: int = 0) -> Tensor:
         """Create positional encoding.
 
         Args:
             x (Tensor): Input tensor (time, batch, `*`).
-            left_context_len: (int): Length of cached left context.
+            block_size (int):
 
         Returns:
-            positional embedding, of shape (batch, left_context_len + 2*time-1, `*`).
+            positional embedding, of shape (1, 2*time-1, `*`) or (1, 4*block_size-1, `*`).
         """
-        self.extend_pe(x, left_context_len)
-        x_size_left = x.size(0) + left_context_len
-        # length of positive side: x.size(0) + left_context_len
-        # length of negative side: x.size(0)
+        self.extend_pe(x)
+        if rel_pos == 0:
+            rel_pos = x.size(0)
         pos_emb = self.pe[
             self.pe.size(0) // 2
-            - x_size_left
+            - rel_pos
             + 1 : self.pe.size(0) // 2  # noqa E203
-            + x.size(0),
+            + rel_pos,
             :
         ]
         pos_emb = pos_emb.unsqueeze(0)
@@ -1472,7 +1570,8 @@ class RelPositionMultiheadAttentionWeights(nn.Module):
         self,
         x: Tensor,
         pos_emb: Tensor,
-        key_padding_mask: Optional[Tensor] = None,
+        attn_offsets: Optional[Tensor] = None,
+        all_pad_mask: Optional[Tensor] = None,
         attn_mask: Optional[Tensor] = None,
     ) -> Tensor:
         r"""
@@ -1580,6 +1679,7 @@ class RelPositionMultiheadAttentionWeights(nn.Module):
         assert attn_scores.shape == (num_heads, batch_size, seq_len, seq_len)
 
         if attn_mask is not None:
+            assert attn_mask is None
             assert attn_mask.dtype == torch.bool
             # use -1000 to avoid nan's where attn_mask and key_padding_mask make
             # all scores zero.  It's important that this be large enough that exp(-1000)
@@ -1587,12 +1687,10 @@ class RelPositionMultiheadAttentionWeights(nn.Module):
             # compares the final weights with zero.
             attn_scores = attn_scores.masked_fill(attn_mask, -1000)
 
-        if key_padding_mask is not None:
-            assert key_padding_mask.shape == (batch_size, seq_len), key_padding_mask.shape
-            attn_scores = attn_scores.masked_fill(
-                key_padding_mask.unsqueeze(1),
-                -1000,
-            )
+        if attn_offsets is not None:
+            # attn_offsets: (1, batch_size, 1, seq_len)
+            # or (1, new_batch_size, 1, kernel)
+            attn_scores = attn_scores + attn_offsets
 
         # We use our own version of softmax, defined in scaling.py, which should
         # save a little of the memory used in backprop by, if we are in
@@ -1600,6 +1698,189 @@ class RelPositionMultiheadAttentionWeights(nn.Module):
         # half-precision output for backprop purposes.
         attn_weights = softmax(attn_scores, dim=-1)
 
+        if all_pad_mask is not None:
+            # For the blocks are all padding
+            # all_pad_mask: (1, new_batch_size, 1, 1)
+            attn_weights = attn_weights.masked_fill(all_pad_mask, 0.0)
+
+        if torch.jit.is_scripting() or torch.jit.is_tracing():
+            pass
+        elif random.random() < 0.001 and not self.training:
+            self._print_attn_entropy(attn_weights)
+
+        attn_weights = nn.functional.dropout(
+            attn_weights, p=self.dropout, training=self.training
+        )
+
+        return attn_weights
+
+    def forward_block(
+        self,
+        x: Tensor,
+        pos_emb: Tensor,
+        block_size: int,
+        block_pad: int,
+        key_padding_mask: Optional[Tensor] = None,
+        attn_mask: Optional[Tensor] = None,
+    ) -> Tensor:
+        r"""
+        Args:
+            x: input of shape (seq_len, batch_size, embed_dim)
+            pos_emb: Positional embedding tensor, of shape (1, 4*block_size-1, pos_dim)
+            block_size: size of block
+            block_pad: pad size at each side of block
+            key_padding_mask: a bool tensor of shape (batch_size, seq_len).  Positions that
+               are True in this mask will be ignored as sources in the attention weighting.
+            attn_mask: mask of shape (seq_len, seq_len) or (batch_size, seq_len, seq_len),
+               interpreted as ([batch_size,] tgt_seq_len, src_seq_len)
+               saying which positions are allowed to attend to which other positions.
+        Returns:
+           a tensor of attention weights, of shape (hum_heads, batch_size * num_blocks, block_size, block_size * 3)
+           interpreted as (hum_heads, batch_size * num_blocks, tgt_seq_len, src_seq_len),
+           where num_blocks = (seq_len + block_size - 1) // block_size.
+        """
+        assert attn_mask is None, "Not supported yet"
+
+        x = self.in_proj(x)
+        query_head_dim = self.query_head_dim
+        pos_head_dim = self.pos_head_dim
+        num_heads = self.num_heads
+
+        seq_len, batch_size, _ = x.shape
+
+        query_dim = query_head_dim * num_heads
+
+        # self-attention
+        q = x[...,0:query_dim]
+        k = x[...,query_dim:2*query_dim]
+        # p is the position-encoding query
+        p = x[...,2*query_dim:]
+        assert p.shape[-1] == num_heads * pos_head_dim
+
+        q = self.copy_query(q)  # for diagnostics only, does nothing.
+        k = self.whiten_keys(self.balance_keys(k))  # does nothing in the forward pass.
+        p = self.copy_pos_query(p)  # for diagnostics only, does nothing.
+
+        # divide into blocks by unfold function
+        num_blocks = (seq_len + block_size - 1) // block_size
+        pad_len = num_blocks * block_size - seq_len
+
+        # (kernel, batch_size * num_blocks, channel)
+        q_blocks = unfold(q, pad_len, num_blocks, kernel=block_size, stride=block_size, padding=0)
+        p_blocks = unfold(p, pad_len, num_blocks, kernel=block_size, stride=block_size, padding=0)
+        k_blocks = unfold(k, pad_len, num_blocks, kernel=block_size + 2 * block_pad, stride=block_size, padding=block_pad)
+
+        # time1 refers to target, time2 refers to source.
+        time1 = q_blocks.size(0)
+        time2 = k_blocks.size(0)
+        new_batch_size = batch_size * num_blocks
+
+        q_blocks = q_blocks.reshape(time1, new_batch_size, num_heads, query_head_dim)
+        p_blocks = p_blocks.reshape(time1, new_batch_size, num_heads, pos_head_dim)
+        k_blocks = k_blocks.reshape(time2, new_batch_size, num_heads, query_head_dim)
+
+        q_blocks = q_blocks.permute(2, 1, 0, 3)  # (head, new_batch, time1, query_head_dim)
+        p_blocks = p_blocks.permute(2, 1, 0, 3)  # (head, new_batch, time1, pos_head_dim)
+        k_blocks = k_blocks.permute(2, 1, 3, 0)  # (head, new_batch, d_k, time2)
+
+        # (head, new_batch, time1, time2)
+        attn_scores = torch.matmul(q_blocks, k_blocks)
+
+        use_pos_scores = False
+        if torch.jit.is_scripting() or torch.jit.is_tracing():
+            # We can't put random.random() in the same line
+            use_pos_scores = True
+        elif not self.training or random.random() >= float(self.pos_emb_skip_rate):
+            use_pos_scores = True
+
+        if use_pos_scores:
+            pos_emb = self.linear_pos(pos_emb)
+            pos_emb = pos_emb.reshape(1, time1 + time2 - 1, num_heads, pos_head_dim).permute(2, 0, 3, 1)
+            # pos shape now: (head, 1, pos_dim, time1+time2-1)
+
+            # (head, batch, time1, pos_dim) x (head, 1, pos_dim, time1+time2-1) -> (head, batch, time1, time1+time2-1)
+            #  [where seq_len2 represents relative position.]
+            pos_scores = torch.matmul(p_blocks, pos_emb)
+            # the following .as_strided() expression converts the last axis of pos_scores from relative
+            # to absolute position.  I don't know whether I might have got the time-offsets backwards or
+            # not, but let this code define which way round it is supposed to be.
+            pos_scores = pos_scores.as_strided((num_heads, new_batch_size, time1, time2),
+                                               (pos_scores.stride(0),
+                                                pos_scores.stride(1),
+                                                pos_scores.stride(2)-pos_scores.stride(3),
+                                                pos_scores.stride(3)),
+                                               storage_offset=pos_scores.stride(3) * (time1 - 1))
+
+            attn_scores = attn_scores + pos_scores
+
+        if torch.jit.is_scripting() or torch.jit.is_tracing():
+            pass
+        elif self.training and random.random() < 0.1:
+            # This is a harder way of limiting the attention scores to not be
+            # too large.  It incurs a penalty if any of them has an absolute
+            # value greater than 50.0.  this should be outside the normal range
+            # of the attention scores.  We use this mechanism instead of, say,
+            # something added to the loss function involving the entropy,
+            # because once the entropy gets very small gradients through the
+            # softmax can become very small, and we'd get zero derivatives.  The
+            # choices of 1.0e-04 as the scale on the penalty makes this
+            # mechanism vulnerable to the absolute scale of the loss function,
+            # but we view this as a failsafe to avoid "implausible" parameter
+            # values rather than a regularization method that should be active
+            # under normal circumstances.
+            attn_scores = penalize_abs_values_gt(attn_scores,
+                                                 limit=25.0,
+                                                 penalty=1.0e-04,
+                                                 name=self.name)
+
+        assert attn_scores.shape == (num_heads, new_batch_size, time1, time2)
+
+        assert attn_mask is None
+        if attn_mask is not None:
+            # TODO:
+            assert attn_mask.dtype == torch.bool
+            # use -1000 to avoid nan's where attn_mask and key_padding_mask make
+            # all scores zero.  It's important that this be large enough that exp(-1000)
+            # is exactly zero, for reasons related to const_attention_rate, it
+            # compares the final weights with zero.
+            attn_scores = attn_scores.masked_fill(attn_mask, -1000)
+
+        # Used to mask out the padding positions
+        attn_offsets = torch.ones(batch_size, seq_len, device=x.device)
+
+        if key_padding_mask is not None:
+            assert key_padding_mask.shape == (batch_size, seq_len), key_padding_mask.shape
+            attn_offsets = attn_offsets.masked_fill(key_padding_mask, 0.0)  # 0 at padding positions
+
+        # (seq_len, batch, 1)
+        attn_offsets = attn_offsets.transpose(0, 1).unsqueeze(-1)
+        # (time2, new_batch_size)
+        attn_offsets = unfold(
+            attn_offsets, pad_len, num_blocks,
+            kernel=block_size + 2 * block_pad, stride=block_size, padding=block_pad,
+        ).squeeze(-1)
+
+        # Used for the blocks are all padding
+        all_pad_mask = (attn_offsets.sum(dim=0, keepdim=True) == 0)   # (1, new_batch_size)
+        all_pad_mask = all_pad_mask.unsqueeze(-1).unsqueeze(-1)  # (1, new_batch_size, 1, 1)
+
+        attn_offsets = 1 - attn_offsets  # 1 at padding positions
+        attn_offsets[attn_offsets != 0] = -1000
+
+        # (1, new_batch_size, 1, time2)
+        attn_offsets = attn_offsets.transpose(0, 1).unsqueeze(1).unsqueeze(0)
+
+        attn_scores = attn_scores + attn_offsets
+
+        # We use our own version of softmax, defined in scaling.py, which should
+        # save a little of the memory used in backprop by, if we are in
+        # automatic mixed precision mode (amp / autocast), by only storing the
+        # half-precision output for backprop purposes.
+        attn_weights = softmax(attn_scores, dim=-1)
+
+        # For the blocks are all padding
+        attn_weights = attn_weights.masked_fill(all_pad_mask, 0.0)
+
         if torch.jit.is_scripting() or torch.jit.is_tracing():
             pass
         elif random.random() < 0.001 and not self.training:
@@ -1678,7 +1959,7 @@ class RelPositionMultiheadAttentionWeights(nn.Module):
         # (head, batch, time1, pos_dim) x (head, 1, pos_dim, seq_len2) -> (head, batch, time1, seq_len2)
         #  [where seq_len2 represents relative position.]
         pos_scores = torch.matmul(p, pos_emb)
-        
+
         if torch.jit.is_tracing():
             (num_heads, batch_size, time1, n) = pos_scores.shape
             rows = torch.arange(start=time1 - 1, end=-1, step=-1)
@@ -1794,6 +2075,63 @@ class SelfAttention(nn.Module):
 
         return x
 
+    def forward_block(
+        self,
+        x: Tensor,
+        attn_weights: Tensor,
+        block_size: int,
+        block_pad: int,
+    ) -> Tensor:
+        """
+        Args:
+          x: input tensor, of shape (seq_len, batch_size, embed_dim)
+         attn_weights: a tensor of attention weights, of shape
+           (hum_heads, batch_size * num_blocks, block_size, block_size * 3)
+           interpreted as (hum_heads, batch_size * num_blocks, tgt_seq_len, src_seq_len),
+           where num_blocks = (seq_len + block_size - 1) // block_size.
+           Expect attn_weights.sum(dim=-1) == 1.
+         block_size: size of block
+         block_pad: pad size at each side of block
+        Returns:
+           a tensor with the same shape as x.
+        """
+        (seq_len, batch_size, embed_dim) = x.shape
+        num_heads = attn_weights.shape[0]
+
+        # divide into blocks by unfold function
+        num_blocks = (seq_len + block_size - 1) // block_size
+        pad_len = num_blocks * block_size - seq_len
+        new_batch_size = batch_size * num_blocks
+        time1 = block_size  # target length
+        time2 = block_size + 2 * block_pad  # source length
+
+        assert attn_weights.shape == (num_heads, new_batch_size, time1, time2)
+
+        x = self.in_proj(x)  # (seq_len, batch_size, num_heads * value_head_dim)
+
+        # (time2, new_batch_size, channel)
+        x_blocks = unfold(x, pad_len, num_blocks, kernel=time2, stride=block_size, padding=block_pad)
+
+        x_blocks = x_blocks.reshape(time2, new_batch_size, num_heads, -1).permute(2, 1, 0, 3)
+        # now x: (num_heads, new_batch_size, time2, value_head_dim)
+        value_head_dim = x_blocks.shape[-1]
+
+        # todo: see whether there is benefit in overriding matmul
+        x = torch.matmul(attn_weights, x_blocks)
+        # v: (num_heads, new_batch_size, time1, value_head_dim)
+
+        x = x.reshape(num_heads, batch_size, num_blocks, time1, value_head_dim)
+        x = x.permute(2, 3, 1, 0, 4).contiguous().view(
+            num_blocks * time1, batch_size, num_heads * value_head_dim)
+
+        x = x[:seq_len]  # (seq_len, batch_size, value_dim)
+
+        # returned value is of shape (seq_len, batch_size, embed_dim), like the input.
+        x = self.out_proj(x)
+        x = self.whiten(x)
+
+        return x
+
     def streaming_forward(
         self,
         x: Tensor,
@@ -1981,6 +2319,78 @@ attn_weights: a Tensor of shape (num_heads, batch_size, seq_len, seq_len)
         x = self.whiten2(x)
         return x
 
+    def forward_block(
+        self,
+        x: Tensor,
+        attn_weights: Tensor,
+        block_size: int,
+        block_pad: int,
+    ) -> Tensor:
+        """.
+        Args:
+           x: a Tensor of shape (seq_len, batch_size, num_channels)
+         attn_weights: a tensor of attention weights, of shape
+           (hum_heads, batch_size * num_blocks, block_size, block_size * 3)
+           interpreted as (hum_heads, batch_size * num_blocks, tgt_seq_len, src_seq_len),
+           where num_blocks = (seq_len + block_size - 1) // block_size.
+           Expect attn_weights.sum(dim=-1) == 1.
+         block_size: size of block
+         block_pad: pad size at each side of block
+        Returns:
+           a Tensor with the same shape as x
+        """
+        x = self.in_proj(x)
+
+        (seq_len, batch_size, _) = x.shape
+        hidden_channels = self.hidden_channels
+
+        s, x, y = x.chunk(3, dim=-1)
+
+        # s will go through tanh.
+
+        s = self.balancer(s)
+        s = self.tanh(s)
+
+        s = s.unsqueeze(-1).reshape(seq_len, batch_size, hidden_channels)
+        x = self.whiten1(x)
+        x = x * s
+        x = self.identity1(x)  # diagnostics only, it's the identity.
+
+        (seq_len, batch_size, embed_dim) = x.shape
+        num_heads = attn_weights.shape[0]
+
+        # divide into blocks by unfold function
+        num_blocks = (seq_len + block_size - 1) // block_size
+        pad_len = num_blocks * block_size - seq_len
+        new_batch_size = batch_size * num_blocks
+        time1 = block_size  # target length
+        time2 = block_size + 2 * block_pad  # source length
+
+        assert attn_weights.shape == (num_heads, new_batch_size, time1, time2)
+
+        # (time2, new_batch_size, channel)
+        x_blocks = unfold(x, pad_len, num_blocks, kernel=time2, stride=block_size, padding=block_pad)
+
+        x_blocks = x_blocks.reshape(time2, new_batch_size, num_heads, -1).permute(2, 1, 0, 3)
+        # now x: (num_heads, new_batch_size, time2, head_dim)
+
+        x = torch.matmul(attn_weights, x_blocks)
+        # now x: (num_heads, new_batch_size, time1, head_dim)
+
+        x = x.reshape(num_heads, batch_size, num_blocks, time1, -1)
+        x = x.permute(2, 3, 1, 0, 4).contiguous().view(
+            num_blocks * time1, batch_size, embed_dim)
+
+        x = x[:seq_len]  # (seq_len, batch_size, embed_dim)
+
+        y = self.identity2(y)
+        x = x * y
+        x = self.identity3(x)
+
+        x = self.out_proj(x)
+        x = self.whiten2(x)
+        return x
+
     def streaming_forward(
         self,
         x: Tensor,
@@ -2220,30 +2630,38 @@ class ScalarMultiply(nn.Module):
 
 
 def _test_zipformer_main(causal: bool = False):
-    batch_size = 5
-    seq_len = 20
     # Just make sure the forward pass runs.
 
+    from icefall.utils import make_pad_mask
+
     c = Zipformer2(
         encoder_dim=(64, 96), encoder_unmasked_dim=(48, 64), num_heads=(4, 4),
+        downsampling_factor=(1, 2),
+        max_block_size=14,
+        block_pad=2,
         causal=causal,
         chunk_size=(4,) if causal else (-1,),
         left_context_frames=(64,)
     )
-    batch_size = 5
-    seq_len = 20
+    batch_size = 2
+    seq_len = 27
+
     # Just make sure the forward pass runs.
-    f = c(
-        torch.randn(seq_len, batch_size, 64),
-        torch.full((batch_size,), seq_len, dtype=torch.int64),
-    )
+    x = torch.randn(seq_len, batch_size, 64)
+    lengths = torch.full((batch_size,), seq_len, dtype=torch.int64)
+    lengths[-1] = 1
+    src_key_padding_mask = make_pad_mask(lengths)
+    f = c(x, lengths, src_key_padding_mask)
     f[0].sum().backward()
     c.eval()
-    f = c(
-        torch.randn(seq_len, batch_size, 64),
-        torch.full((batch_size,), seq_len, dtype=torch.int64),
-    )
+
+    x = torch.randn(seq_len, batch_size, 64)
+    lengths = torch.full((batch_size,), seq_len, dtype=torch.int64)
+    lengths[-1] = seq_len - 2
+    src_key_padding_mask = make_pad_mask(lengths)
+    f = c(x, lengths, src_key_padding_mask)
     f  # to remove flake8 warnings
+    print(f[0].sum())
 
 
 if __name__ == "__main__":
@@ -2251,4 +2669,4 @@ if __name__ == "__main__":
     torch.set_num_threads(1)
     torch.set_num_interop_threads(1)
     _test_zipformer_main(False)
-    _test_zipformer_main(True)
+    # _test_zipformer_main(True)