[PATCH] csum-file: flush less often

From: Derrick Stolee <dstolee@microsoft.com>

In an independent investigation, I noticed that do_write_index() in read-cache.c has its own hashing logic and buffering mechanism. Specifically, the ce_write() method was introduced by 4990aadc (Speed up index file writing by chunking it nicely, 2005-04-20) and similar mechanisms were introduced a few months later in c38138cd (git-pack-objects: write the pack files with a SHA1 csum, 2005-06-26). Based on the timing, in the early days of the Git codebase, I figured that these roughly equivalent code paths were never unified only because it got lost in the shuffle. The hashfile API has since been used extensively in other file formats, such as pack-indexes, mult-pack-indexes, and commit-graphs. Therefore, it seems prudent to unify the index writing code to use the same mechanism.

However, upon doing that refactoring process, I noticed that this caused some commands that write the index to slow down by 1-2%! I then looked for a reason why this could be.

First, I noticed that the mechanisms use different buffer sizes. The hashfile uses an 8KB buffer while the index uses an 128KB buffer. Testing with a variety of different buffer sizes made little difference.

Next, I inspected the buffering code itself, and found an important difference. Specifically, every call to hashwrite() was causing a flush of the filestream, even if it was a very small write. With many callers using helpers like hashwrite_be32() to write integers in network-byte order, this was leading to many more file flushes than necessary.

This change modifies hashwrite() to always populate the hashfile buffer, and only flush when that buffer is full. This is safe to do because all consumers of a hashfile must call finalize_hashfile(), which flushes the buffer at the start.

It is worth noting that this is modifying logic introduced by a8032d12 (sha1write: don't copy full sized buffers, 2008-09-02) which reduces memcpy() calls when the input buffer is sufficiently longer than the hashfile's buffer, causing nr to be the length of the full buffer. Use the input buffer directly in these cases. Since we don't guarantee that the buffer is flushed by the end of hashwrite(), we need to group some offset logic into the condition that memcpy() is necessary. Note that nr is equal to sizeof(f->buffer) only when f->offset is zero, so that condition does not need to be added here.

As for performance, I focused on known commands that spend a significant amount of time writing through the hashfile API, especially if using small buffers as in hashwrite_be32(). 'git multi-pack-index write' was an excellent example (deleting the multi-pack-index file between runs) and demonstrated this performance change in the Linux kernal repo:

Benchmark #1: old
  Time (mean ± σ):      2.229 s ±  0.143 s    [User: 1.409 s, System: 0.327 s]
  Range (min … max):    2.160 s …  2.636 s    10 runs

Benchmark #2: new
  Time (mean ± σ):      2.162 s ±  0.005 s    [User: 1.392 s, System: 0.323 s]
  Range (min … max):    2.152 s …  2.172 s    10 runs

Summary
  'new' ran
    1.03 ± 0.07 times faster than 'old'

Similarly, the same command on the Git repository gave these numbers:

Benchmark #1: old
  Time (mean ± σ):     230.5 ms ±   6.3 ms    [User: 140.5 ms, System: 42.9 ms]
  Range (min … max):   221.7 ms … 240.6 ms    12 runs

Benchmark #2: new
  Time (mean ± σ):     220.6 ms ±   5.1 ms    [User: 139.5 ms, System: 34.1 ms]
  Range (min … max):   214.0 ms … 229.0 ms    13 runs

Summary
  'new' ran
    1.05 ± 0.04 times faster than 'old'

Finally, to demonstrate that performance holds when frequently using large buffers, the numbers below are for 'git pack-objects' packing all objects in the Git repository between v2.30.0 and v2.31.1:

Benchmark #1: old
  Time (mean ± σ):      1.003 s ±  0.045 s    [User: 1.877 s, System: 0.167 s]
  Range (min … max):    0.931 s …  1.044 s    10 runs

Benchmark #2: new
  Time (mean ± σ):     976.4 ms ±  42.2 ms    [User: 1.854 s, System: 0.192 s]
  Range (min … max):   940.1 ms … 1049.3 ms    10 runs

Summary
  'new' ran
    1.03 ± 0.06 times faster than 'old'

With these consistent improvements of 3-5%, it will be possible to move the index writing logic over to hashfile without performance degradation.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
    csum-file: flush less often
    
    I found this while poking around the index.
    
    Thanks, -Stolee

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-914%2Fderrickstolee%2Fhashfile-flush-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-914/derrickstolee/hashfile-flush-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/914

 csum-file.c | 22 ++++++++--------------
 1 file changed, 8 insertions(+), 14 deletions(-)