[PATCH] read-cache: make the index write buffer size 128K

From: Neeraj K. Singh via GitGitGadget <gitgitgadget@gmail.com>
Date: Feb 18, 2021, 02:48 UTC
Message-ID: <pull.877.git.1613616506949.gitgitgadget@gmail.com>

From: Neeraj Singh <neerajsi@ntdev.microsoft.com>

Writing an index 8K at a time invokes the OS filesystem and caching code very frequently, introducing noticeable overhead while writing large indexes. When experimenting with different write buffer sizes on Windows writing the Windows OS repo index (260MB), most of the benefit came by bumping the index write buffer size to 64K. I picked 128K to ensure that we're past the knee of the curve.

With this change, the time under do_write_index for an index with 3M files goes from ~1.02s to ~0.72s.

Signed-off-by: Neeraj Singh <neerajsi@ntdev.microsoft.com>
---
    read-cache: make the index write buffer size 128K
    
    Writing an index 8K at a time invokes the OS filesystem and caching code
    very frequently, introducing noticeable overhead while writing large
    indexes. When experimenting with different write buffer sizes on Windows
    writing the Windows OS repo index (260MB), most of the benefit came by
    bumping the index write buffer size to 64K. I picked 128K to ensure that
    we're past the knee of the curve.
    
    With this change, the time under do_write_index for an index with 3M
    files goes from ~1.02s to ~0.72s.
    
    Signed-off-by: Neeraj Singh neerajsi@ntdev.microsoft.com
    
    Note: This was previously discussed on the mailing list in 2016 at:
    https://lore.kernel.org/git/1458350341-12276-1-git-send-email-dturner@twopensource.com/.
    
    Since then, I believe we have a couple changes:
    
     * 'small' development platforms like raspberry pi have gotten larger
       (4GB RAM).
     * spectre and meltdown make individual system calls more expensive when
       mitigations are enabled
     * there have been many investments to make very large repos scale well
       in git, so huge repos are more common now.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-877%2Fneerajsi-msft%2Fneerajsi%2Findex-buffer-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-877/neerajsi-msft/neerajsi/index-buffer-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/877

 read-cache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/read-cache.c b/read-cache.c
index 29144cf879e7..a5b2779b9586 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2447,7 +2447,7 @@ int repo_index_has_changes(struct repository *repo,
 	}
 }
 
-#define WRITE_BUFFER_SIZE 8192
+#define WRITE_BUFFER_SIZE (128 * 1024)
 static unsigned char write_buffer[WRITE_BUFFER_SIZE];
 static unsigned long write_buffer_len;
 

base-commit: 45526154a57d15947cad7262230d0b935cedb9d3

-- 
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