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

[Hui Zhu]

On 2019/11/9 12:04 äå, Dan Streetman wrote:
> On Fri, Oct 25, 2019 at 4:02 AM Hui Zhu <teawaterz@xxxxxxxxxxxxxxxxx> 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?

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@xxxxxxxxxxxxxxxxx>
> > ---
> >   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