[PATCH v2] index-pack: retain child bases in delta cache

From: Arijit Banerjee <arijit@effectiveailabs.com>

When resolving a delta whose result has children of its own, index-pack adds the result to work_head, accounts its data in base_cache_used, and calls prune_base_data(). It then immediately frees that same data.

This bypasses the existing delta base cache policy and can force later descendants to reconstruct the queued base again. Let the existing delta_base_cache_limit pruning policy decide whether to keep or evict the data instead.

This does not add a new cache or increase the cache limit. The object data is already accounted in base_cache_used before prune_base_data() runs, and the existing pruning and base cleanup paths still release it.

On a quiet Ubuntu 24.04 VM with 16 vCPUs, 32 GiB RAM, and local SSD, direct index-pack timings on single-pack Linux fixtures improved as follows:

  linux blobless: 69.17s -> 57.98s (16.2% faster), RSS flat
  linux full:     280.72s -> 236.32s (15.8% faster), RSS +1.9%

Five-repeat medians on public repositories also improved:

  git.git:  12.31s -> 10.70s (13.1% faster)
  libgit2:   3.35s ->  2.88s (14.0% faster)
  redis:     6.52s ->  5.64s (13.5% faster)
  cpython:  33.02s -> 31.44s (4.8% faster)

The standard p5302 perf test on a smaller git.git fixture was neutral:

  5302.9 index-pack default threads:
    11.21(38.07+1.33) -> 11.16(37.90+1.31), -0.4%

t/t5302-pack-index.sh passed, and GitGitGadget's linux-leaks CI also exercised that test under SANITIZE=leak.

Signed-off-by: Arijit Banerjee <arijit@effectiveailabs.com>
---
    index-pack: retain child bases in delta cache
    
    Speed up the local pack indexing phase of clone/fetch for large
    delta-compressed packs by keeping reconstructed delta bases available
    for reuse when they are queued for later delta resolution.
    
    When index-pack reconstructs a child base and queues it for resolving
    descendant deltas, it currently frees that data immediately. This can
    force the same base to be reconstructed again. Instead, keep it in the
    existing delta base cache and let the existing delta_base_cache_limit
    policy decide whether to retain or evict it.
    
    This does not add a new cache or increase the cache limit. The object
    data is already accounted in base_cache_used, and prune_base_data() is
    already called at this point.
    
    Correctness:
    
     * t/t5302-pack-index.sh passed all 36 tests.
    
    Benchmarks on a quiet Ubuntu 24.04 VM, 16 vCPU, 32 GiB RAM, local SSD:
    
    pack baseline patched wall-time change RSS change linux blobless 69.17s
    57.98s 16.2% faster -0.0% linux full 280.72s 236.32s 15.8% faster +1.9%
    
    Five-repeat public-repo medians also improved: git.git 13.1%, libgit2
    14.0%, redis 13.5%, cpython 4.8%.
    
    Perf on the linux blobless pack showed the same direction under
    profiling: 76.64s baseline vs 61.09s patched, with similar RSS.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2131%2Farijit91%2Findex-pack-retain-child-base-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2131/arijit91/index-pack-retain-child-base-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/2131

Range-diff vs v1:

 1:  cafb1700be ! 1:  42eca38f51 index-pack: retain child bases in delta cache
     @@ Commit message
      
          When resolving a delta whose result has children of its own,
          index-pack adds the result to work_head, accounts its data in
     -    base_cache_used, and calls prune_base_data(). It then immediately
     -    frees that same data.
     +    base_cache_used, and calls prune_base_data(). It then immediately frees
     +    that same data.
      
          This bypasses the existing delta base cache policy and can force later
          descendants to reconstruct the queued base again. Let the existing
          delta_base_cache_limit pruning policy decide whether to keep or evict
          the data instead.
      
     +    This does not add a new cache or increase the cache limit. The object
     +    data is already accounted in base_cache_used before prune_base_data()
     +    runs, and the existing pruning and base cleanup paths still release it.
     +
     +    On a quiet Ubuntu 24.04 VM with 16 vCPUs, 32 GiB RAM, and local SSD,
     +    direct index-pack timings on single-pack Linux fixtures improved as
     +    follows:
     +
     +      linux blobless: 69.17s -> 57.98s (16.2% faster), RSS flat
     +      linux full:     280.72s -> 236.32s (15.8% faster), RSS +1.9%
     +
     +    Five-repeat medians on public repositories also improved:
     +
     +      git.git:  12.31s -> 10.70s (13.1% faster)
     +      libgit2:   3.35s ->  2.88s (14.0% faster)
     +      redis:     6.52s ->  5.64s (13.5% faster)
     +      cpython:  33.02s -> 31.44s (4.8% faster)
     +
     +    The standard p5302 perf test on a smaller git.git fixture was neutral:
     +
     +      5302.9 index-pack default threads:
     +        11.21(38.07+1.33) -> 11.16(37.90+1.31), -0.4%
     +
     +    t/t5302-pack-index.sh passed, and GitGitGadget's linux-leaks CI also
     +    exercised that test under SANITIZE=leak.
     +
          Signed-off-by: Arijit Banerjee <arijit@effectiveailabs.com>
      
       ## builtin/index-pack.c ##

 builtin/index-pack.c | 1 -
 1 file changed, 1 deletion(-)