Re: [PATCH v4] mm: swap: async free swap slot cache entries

[Chris Li]

On Thu, Feb 15, 2024 at 10:31 AM Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote:
>
> On Wed, 2024-02-14 at 17:02 -0800, Chris Li wrote:
> > We discovered that 1% swap page fault is 100us+ while 50% of
> > the swap fault is under 20us.
> >
> > Further investigation shows that a large portion of the time
> > spent in the free_swap_slots() function for the long tail case.
> >
> > The percpu cache of swap slots is freed in a batch of 64 entries
> > inside free_swap_slots(). These cache entries are accumulated
> > from previous page faults, which may not be related to the current
> > process.
> >
> > Doing the batch free in the page fault handler causes longer
> > tail latencies and penalizes the current process.
> >
> > When the swap cache slot is full, schedule async free cached
> > swap slots in a work queue, before the next swap fault comes in.
> > If the next swap fault comes in very fast, before the async
> > free gets a chance to run. It will directly free all the swap
> > cache in the swap fault the same way as previously.
> >
> > Testing:
> >
> > Chun-Tse did some benchmark in chromebook, showing that
> > zram_wait_metrics improve about 15% with 80% and 95% confidence.
> >
> > I recently ran some experiments on about 1000 Google production
> > machines. It shows swapin latency drops in the long tail
> > 100us - 500us bucket dramatically.
> >
> > platform      (100-500us)             (0-100us)
> > A             1.12% -> 0.36%          98.47% -> 99.22%
> > B             0.65% -> 0.15%          98.96% -> 99.46%
> > C             0.61% -> 0.23%          98.96% -> 99.38%
> >
> > Signed-off-by: Chris Li <chrisl@xxxxxxxxxx>
>
> Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>

Thank you so much for your review.

Chris