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Code Issues Releases Activity

zstd: don't call zstd_mempool_reap if there are no buffers (#16302)

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zfs_zstd_cache_reap_now is issued every second.

zstd_mempool_reap checks for both pool existence and buffer count, but
that's still 2 func calls which are trivially avoidable.

With clang it even avoids pushing the stack pointer (but still suffers
the mispredict due to a forward jump, not modified in case someone is
using zstd):

<+0>:     cmpq   $0x0,0x0(%rip)        # <zfs_zstd_cache_reap_now+8>
<+8>:     je     0x217de4 <zfs_zstd_cache_reap_now+36>
<+10>:    push   %rbp
<+11>:    mov    %rsp,%rbp
<+14>:    mov    0x0(%rip),%rdi        # <zfs_zstd_cache_reap_now+21>
<+21>:    call   0x217df0 <zstd_mempool_reap>
<+26>:    mov    0x0(%rip),%rdi        # <zfs_zstd_cache_reap_now+33>
<+33>:    pop    %rbp
<+34>:    jmp    0x217df0 <zstd_mempool_reap>
<+36>:    ret

Preferably the call would not be made to begin with if zstd is not used,
but this retains all the logic confined to zstd code.

Sponsored by:	Rubicon Communications, LLC ("Netgate")
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
This commit is contained in:
Mateusz Guzik 2024-07-15 23:51:37 +02:00 committed by GitHub
parent c87cb22ba9
commit a7fc4c85e3
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1 changed files with 7 additions and 0 deletions
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7
module/zstd/zfs_zstd.c
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@ -844,6 +844,13 @@ zstd_mempool_deinit(void)
void
zfs_zstd_cache_reap_now(void)
{
/*
* Short-circuit if there are no buffers to begin with.
*/
if (ZSTDSTAT(zstd_stat_buffers) == 0)
return;
/*
* calling alloc with zero size seeks
* and releases old unused objects