On Mon, Mar 16, 2020 at 3:24 PM Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> wrote:
>
> When backporting commit 9c4e6b1a7027 ("mm, mlock, vmscan: no more
> skipping pagevecs") to our 4.9 kernel, our test bench noticed around 10%
> down with a couple of vm-scalability's test cases (lru-file-readonce,
> lru-file-readtwice and lru-file-mmap-read). I didn't see that much down
> on my VM (32c-64g-2nodes). It might be caused by the test configuration,
> which is 32c-256g with NUMA disabled and the tests were run in root memcg,
> so the tests actually stress only one inactive and active lru. It
> sounds not very usual in mordern production environment.
>
> That commit did two major changes:
> 1. Call page_evictable()
> 2. Use smp_mb to force the PG_lru set visible
>
> It looks they contribute the most overhead. The page_evictable() is a
> function which does function prologue and epilogue, and that was used by
> page reclaim path only. However, lru add is a very hot path, so it
> sounds better to make it inline. However, it calls page_mapping() which
> is not inlined either, but the disassemble shows it doesn't do push and
> pop operations and it sounds not very straightforward to inline it.
>
> Other than this, it sounds smp_mb() is not necessary for x86 since
> SetPageLRU is atomic which enforces memory barrier already, replace it
> with smp_mb__after_atomic() in the following patch.
>
> With the two fixes applied, the tests can get back around 5% on that
> test bench and get back normal on my VM. Since the test bench
> configuration is not that usual and I also saw around 6% up on the
> latest upstream, so it sounds good enough IMHO.
>
> The below is test data (lru-file-readtwice throughput) against the v5.6-rc4:
> mainline w/ inline fix
> 150MB 154MB
>
> With this patch the throughput gets 2.67% up. The data with using
> smp_mb__after_atomic() is showed in the following patch.
>
> Fixes: 9c4e6b1a7027 ("mm, mlock, vmscan: no more skipping pagevecs")
> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
So, I tested on a real machine with limiting the 'dd' on a single node
and reading 100 GiB sparse file (less than a single node). I just ran
a single instance to not cause the lru lock contention. The cmd I used
is "dd if=file-100GiB of=/dev/null bs=4k". I ran the cmd 10 times with
drop_caches in between and measured the time it took.
Without patch: 56.64143 +- 0.672 sec
With patches: 56.10 +- 0.21 sec
Reviewed-and-Tested-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
