Re: [PATCH v1 0/8] Batched zswap_store() with compression batching

[Yosry Ahmed <yosry@kernel.org>]

On Sun, Oct 18, 2026 at 06:35:01PM -0700, Kanchana P. Sridhar wrote:
> On Fri, Oct 16, 2026 at 10:07 PM Herbert Xu <herbert@gondor.apana.org.au> wrote:
> >
> > On Fri, Oct 16, 2026 at 10:02:13PM -0700, Kanchana P. Sridhar wrote:
> > [ .. ]
> >
> > That isn't quite right though.  We need to ensure that it really
> > is a singleton before taking the short-cut.  So you need at
> > least one conditional:
> >
> >         if (likely(req->unit_size == req->slen))
> >                 take short-cut
> 
> I added the check for the singleton req [2]. I hope this looks Ok.
> 
> With the latest crypto_acomp_compress(), the batch compress latency
> improves considerably:
> 
> usemem 10 iterations average throughput:
> 
>    patch 6 (store batching only): 19,119 KB/s
> patch 8 (+ compress batching): 24,076 KB/s (+25.93%)

This is exactly what I mean by the numbers varying too much to draw
any conclusions. I am not sure if it's the VM setup or the usemem test
itself, but previously it was argued that the short-circuit in
crypto_acomp_compress() was ~10% faster than the one in
acomp_do_req_chain() (20,946 vs 19,058 KB/s) due to avoiding a function
call and two conditionals, and now *adding* a conditional improves
performance by 15% (20,946 -> 24,076 KB/s)?

Moreover, for zswap with ZSTD the added check is always true: the batch
size is 1, so slen == unit_size == PAGE_SIZE for every request. The
previous version and this one execute the same code, so the 15%
difference can only be noise. With the stddev you reported (~5.4k KB/s
over 10 iterations), both differences are well within the noise.

I think something is wrong with the performance testing methodology
here. The results are self-contradicting and do not make much sense to
me. Did I miss something?

> 
> bpftrace measurement of zswap compress latency per-page:
> 
> Sequential_zswap_compress  nr_samples     p25     p50      p75
> p95       p99
> =================================================================================
> zswap_compress_ns_per_page  1,722,489  43,947  57,791   88,489
> 130,165   323,961
> 
> Batched_zswap_compress     nr_samples     p25     p50      p75
> p95       p99
> =================================================================================
> zswap_compress_ns_per_page    665,267  41,511  54,915   77,707
> 124,648   213,962
> =================================================================================
>  Latency saving (ns)                   -2,435  -2,876  -10,782
> -5,517  -109,999
> =================================================================================

I measured zswap_store() (not zswap_compress()), to get the per-folio
latency, and it didn't seem to change much beyond noise in my testing. I
also notice that the number of samples is very different between cases.