linux-kernel - Re: [PATCH] zswap: Add shrink_enabled that can disable swap shrink to increase store performance

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Date:   Sun, 26 Jan 2020 14:36:19 -0500
From:   Dan Streetman <ddstreet@...e.org>
To:     Hui Zhu <teawaterz@...ux.alibaba.com>,
        Vitaly Wool <vitalywool@...il.com>
Cc:     Seth Jennings <sjenning@...hat.com>, Linux-MM <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] zswap: Add shrink_enabled that can disable swap shrink to
 increase store performance

On Sun, Nov 10, 2019 at 8:58 PM Hui Zhu <teawaterz@...ux.alibaba.com> wrote:
>
>
>
> On 2019/11/9 12:04 上午, Dan Streetman wrote:
> > On Fri, Oct 25, 2019 at 4:02 AM Hui Zhu <teawaterz@...ux.alibaba.com> wrote:
> >>
> >> zswap will try to shrink pool when zswap is full.
> >> This commit add shrink_enabled that can disable swap shrink to increase
> >> store performance.  User can disable swap shrink if care about the store
> >> performance.
> >
> > I don't understand - if zswap is full it can't store any more pages
> > without shrinking the current pool.  This commit will just force all
> > pages to swap when zswap is full.  This has nothing to do with 'store
> > performance'.
> >
> > I think it would be much better to remove any user option for this and
> > implement some hysteresis; store pages normally until the zpool is
> > full, then reject all pages going to that pool until there is some %
> > free, at which point allow pages to be stored into the pool again.
> > That will prevent (or at least reduce) the constant performance hit
> > when a zpool fills up, and just fallback to normal swapping to disk
> > until the zpool has some amount of free space again.
> >
>
> This idea is really cool!
> Do you mind I make a patch for it?

Sorry for the delay, again.

I think Vitaly has a patch adding this.

>
> Thanks,
> Hui
>
> >>
> >> For example in a VM with 1 CPU 1G memory 4G swap:
> >> echo lz4 > /sys/module/zswap/parameters/compressor
> >> echo z3fold > /sys/module/zswap/parameters/zpool
> >> echo 0 > /sys/module/zswap/parameters/same_filled_pages_enabled
> >> echo 1 > /sys/module/zswap/parameters/enabled
> >> usemem -a -n 1 $((4000 * 1024 * 1024))
> >> 4718592000 bytes / 114937822 usecs = 40091 KB/s
> >> 101700 usecs to free memory
> >> echo 0 > /sys/module/zswap/parameters/shrink_enabled
> >> usemem -a -n 1 $((4000 * 1024 * 1024))
> >> 4718592000 bytes / 8837320 usecs = 521425 KB/s
> >> 129577 usecs to free memory
> >>
> >> The store speed increased when zswap shrink disabled.
> >>
> >> Signed-off-by: Hui Zhu <teawaterz@...ux.alibaba.com>
> >> ---
> >>   mm/zswap.c | 7 +++++++
> >>   1 file changed, 7 insertions(+)
> >>
> >> diff --git a/mm/zswap.c b/mm/zswap.c
> >> index 46a3223..731e3d1e 100644
> >> --- a/mm/zswap.c
> >> +++ b/mm/zswap.c
> >> @@ -114,6 +114,10 @@ static bool zswap_same_filled_pages_enabled = true;
> >>   module_param_named(same_filled_pages_enabled, zswap_same_filled_pages_enabled,
> >>                     bool, 0644);
> >>
> >> +/* Enable/disable zswap shrink (enabled by default) */
> >> +static bool zswap_shrink_enabled = true;
> >> +module_param_named(shrink_enabled, zswap_shrink_enabled, bool, 0644);
> >> +
> >>   /*********************************
> >>   * data structures
> >>   **********************************/
> >> @@ -947,6 +951,9 @@ static int zswap_shrink(void)
> >>          struct zswap_pool *pool;
> >>          int ret;
> >>
> >> +       if (!zswap_shrink_enabled)
> >> +               return -EPERM;
> >> +
> >>          pool = zswap_pool_last_get();
> >>          if (!pool)
> >>                  return -ENOENT;
> >> --
> >> 2.7.4
> >>