reftable/block: reuse `zstream` state on inflation

When calling `inflateInit()` and `inflate()`, the zlib library will
allocate several data structures for the underlying `zstream` to keep
track of various information. Thus, when inflating repeatedly, it is
possible to optimize memory allocation patterns by reusing the `zstream`
and then calling `inflateReset()` on it to prepare it for the next chunk
of data to inflate.

This is exactly what the reftable code is doing: when iterating through
reflogs we need to potentially inflate many log blocks, but we discard
the `zstream` every single time. Instead, as we reuse the `block_reader`
for each of the blocks anyway, we can initialize the `zstream` once and
then reuse it for subsequent inflations.

Refactor the code to do so, which leads to a significant reduction in
the number of allocations. The following measurements were done when
iterating through 1 million reflog entries. Before:

  HEAP SUMMARY:
      in use at exit: 13,473 bytes in 122 blocks
    total heap usage: 23,028 allocs, 22,906 frees, 162,813,552 bytes allocated

After:

  HEAP SUMMARY:
      in use at exit: 13,473 bytes in 122 blocks
    total heap usage: 302 allocs, 180 frees, 88,352 bytes allocated

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/reftable/block.c b/reftable/block.c
index 435922b569ccca9ef7d4a17931b970110796638b..c6c4a68ea147522dc7886d3f61f85af94fcf2ad4 100644 (file)
--- a/reftable/block.c
+++ b/reftable/block.c
@@ -198,7 +198,6 @@ int block_reader_init(struct block_reader *br, struct reftable_block *block,
                uint32_t block_header_skip = 4 + header_off;
                uLong dst_len = sz - block_header_skip;
                uLong src_len = block->len - block_header_skip;
-               z_stream stream = {0};
 
                /* Log blocks specify the *uncompressed* size in their header. */
                REFTABLE_ALLOC_GROW(br->uncompressed_data, sz,
@@ -207,16 +206,21 @@ int block_reader_init(struct block_reader *br, struct reftable_block *block,
                /* Copy over the block header verbatim. It's not compressed. */
                memcpy(br->uncompressed_data, block->data, block_header_skip);
 
-               err = inflateInit(&stream);
+               if (!br->zstream) {
+                       REFTABLE_CALLOC_ARRAY(br->zstream, 1);
+                       err = inflateInit(br->zstream);
+               } else {
+                       err = inflateReset(br->zstream);
+               }
                if (err != Z_OK) {
                        err = REFTABLE_ZLIB_ERROR;
                        goto done;
                }
 
-               stream.next_in = block->data + block_header_skip;
-               stream.avail_in = src_len;
-               stream.next_out = br->uncompressed_data + block_header_skip;
-               stream.avail_out = dst_len;
+               br->zstream->next_in = block->data + block_header_skip;
+               br->zstream->avail_in = src_len;
+               br->zstream->next_out = br->uncompressed_data + block_header_skip;
+               br->zstream->avail_out = dst_len;
 
                /*
                 * We know both input as well as output size, and we know that
@@ -225,15 +229,14 @@ int block_reader_init(struct block_reader *br, struct reftable_block *block,
                 * here to instruct zlib to inflate the data in one go, which
                 * is more efficient than using `Z_NO_FLUSH`.
                 */
-               err = inflate(&stream, Z_FINISH);
-               inflateEnd(&stream);
+               err = inflate(br->zstream, Z_FINISH);
                if (err != Z_STREAM_END) {
                        err = REFTABLE_ZLIB_ERROR;
                        goto done;
                }
                err = 0;
 
-               if (stream.total_out + block_header_skip != sz) {
+               if (br->zstream->total_out + block_header_skip != sz) {
                        err = REFTABLE_FORMAT_ERROR;
                        goto done;
                }
@@ -242,7 +245,7 @@ int block_reader_init(struct block_reader *br, struct reftable_block *block,
                reftable_block_done(block);
                block->data = br->uncompressed_data;
                block->len = sz;
-               full_block_size = src_len + block_header_skip - stream.avail_in;
+               full_block_size = src_len + block_header_skip - br->zstream->avail_in;
        } else if (full_block_size == 0) {
                full_block_size = sz;
        } else if (sz < full_block_size && sz < block->len &&
@@ -275,6 +278,8 @@ done:
 
 void block_reader_release(struct block_reader *br)
 {
+       inflateEnd(br->zstream);
+       reftable_free(br->zstream);
        reftable_free(br->uncompressed_data);
        reftable_block_done(&br->block);
 }

diff --git a/reftable/block.h b/reftable/block.h
index 12414eb64271230581e4ff446ae3cdc6fdf5b72c..c1bd1892cbec12d16522ee1042f65fbda80aced5 100644 (file)
--- a/reftable/block.h
+++ b/reftable/block.h
@@ -56,6 +56,8 @@ int block_writer_finish(struct block_writer *w);
 /* clears out internally allocated block_writer members. */
 void block_writer_release(struct block_writer *bw);
 
+struct z_stream;
+
 /* Read a block. */
 struct block_reader {
        /* offset of the block header; nonzero for the first block in a
@@ -67,6 +69,7 @@ struct block_reader {
        int hash_size;
 
        /* Uncompressed data for log entries. */
+       z_stream *zstream;
        unsigned char *uncompressed_data;
        size_t uncompressed_cap;
 

diff --git a/reftable/reader.c b/reftable/reader.c
index aacd5f1337ad53c73b7f6ba66b7be7fa1a4b65fb..481dff10d49b2fda49c13156b4f10e0dc0f3d276 100644 (file)
--- a/reftable/reader.c
+++ b/reftable/reader.c
@@ -459,6 +459,7 @@ static int reader_seek_linear(struct table_iter *ti,
                 * we would not do a linear search there anymore.
                 */
                memset(&next.br.block, 0, sizeof(next.br.block));
+               next.br.zstream = NULL;
                next.br.uncompressed_data = NULL;
                next.br.uncompressed_cap = 0;