Re: [PATCH 4/5] mm: zswap: add basic meminfo and vmstat coverage

[Yang Shi <shy828301@gmail.com>]

On Thu, Apr 28, 2022 at 7:49 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Thu, Apr 28, 2022 at 7:36 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > On Wed, Apr 27, 2022 at 04:36:22PM -0700, Shakeel Butt wrote:
> > > On Wed, Apr 27, 2022 at 3:32 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > > >
> > > > On Wed, Apr 27, 2022 at 05:20:31PM -0400, Johannes Weiner wrote:
> > > > > On Wed, Apr 27, 2022 at 01:29:34PM -0700, Minchan Kim wrote:
> > > > > > Hi Johannes,
> > > > > >
> > > > > > On Wed, Apr 27, 2022 at 12:00:15PM -0400, Johannes Weiner wrote:
> > > > > > > Currently it requires poking at debugfs to figure out the size and
> > > > > > > population of the zswap cache on a host. There are no counters for
> > > > > > > reads and writes against the cache. As a result, it's difficult to
> > > > > > > understand zswap behavior on production systems.
> > > > > > >
> > > > > > > Print zswap memory consumption and how many pages are zswapped out in
> > > > > > > /proc/meminfo. Count zswapouts and zswapins in /proc/vmstat.
> > > > > > >
> > > > > > > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > > > > > > ---
> > > > > > >  fs/proc/meminfo.c             |  7 +++++++
> > > > > > >  include/linux/swap.h          |  5 +++++
> > > > > > >  include/linux/vm_event_item.h |  4 ++++
> > > > > > >  mm/vmstat.c                   |  4 ++++
> > > > > > >  mm/zswap.c                    | 13 ++++++-------
> > > > > > >  5 files changed, 26 insertions(+), 7 deletions(-)
> > > > > > >
> > > > > > > diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
> > > > > > > index 6fa761c9cc78..6e89f0e2fd20 100644
> > > > > > > --- a/fs/proc/meminfo.c
> > > > > > > +++ b/fs/proc/meminfo.c
> > > > > > > @@ -86,6 +86,13 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
> > > > > > >
> > > > > > >   show_val_kb(m, "SwapTotal:      ", i.totalswap);
> > > > > > >   show_val_kb(m, "SwapFree:       ", i.freeswap);
> > > > > > > +#ifdef CONFIG_ZSWAP
> > > > > > > + seq_printf(m,  "Zswap:          %8lu kB\n",
> > > > > > > +            (unsigned long)(zswap_pool_total_size >> 10));
> > > > > > > + seq_printf(m,  "Zswapped:       %8lu kB\n",
> > > > > > > +            (unsigned long)atomic_read(&zswap_stored_pages) <<
> > > > > > > +            (PAGE_SHIFT - 10));
> > > > > > > +#endif
> > > > > >
> > > > > > I agree it would be very handy to have the memory consumption in meminfo
> > > > > >
> > > > > > https://lore.kernel.org/all/YYwZXrL3Fu8%2FvLZw@google.com/
> > > > > >
> > > > > > If we really go this Zswap only metric instead of general term
> > > > > > "Compressed", I'd like to post maybe "Zram:" with same reason
> > > > > > in this patchset. Do you think that's better idea instead of
> > > > > > introducing general term like "Compressed:" or something else?
> > > > >
> > > > > I'm fine with changing it to Compressed. If somebody cares about a
> > > > > more detailed breakdown, we can add Zswap, Zram subsets as needed.
> > > >
> > > > It does raise the question what to do about cgroup, though. Should the
> > > > control files (memory.zswap.current & memory.zswap.max) apply to zram
> > > > in the future? If so, we should rename them, too.
> > > >
> > > > I'm not too familiar with zram, maybe you can provide some
> > > > background. AFAIU, Google uses zram quite widely; all the more
> > > > confusing why there is no container support for it yet.
> > > >
> > > > Could you shed some light?
> > > >
> > >
> > > I can shed light on the datacenter workloads. We use cgroup (still on
> > > v1) and zswap. For the workloads/applications, the swap (or zswap) is
> > > transparent in the sense that they are charged exactly the same
> > > irrespective of how much their memory is zswapped-out. Basically the
> > > applications see the same usage which is actually v1's
> > > memsw.usage_in_bytes. We dynamically increase the swap size if it is
> > > low, so we are not really worried about one job hogging the swap
> > > space.
> > >
> > > Regarding stats we actually do have them internally representing
> > > compressed size and number of pages in zswap. The compressed size is
> > > actually used for OOM victim selection. The memsw or v2's swap usage
> > > in the presence of compression based swap does not actually tell how
> > > much memory can potentially be released by evicting a job. For example
> > > if there are two jobs 'A' and 'B'. Both of them have 100 pages
> > > compressed but A's 100 pages are compressed to let's say 10 pages
> > > while B's 100 pages are compressed to 70 pages. It is preferable to
> > > kill B as that will release 70 pages. (This is a very simplified
> > > explanation of what we actually do).
> >
> > Ah, so zram is really only used by the mobile stuff after all.
> >
> > In the DC, I guess you don't use disk swap in conjunction with zswap,
> > so those writeback cache controls are less interesting to you?
>
> Yes, we have some modifications to zswap to make it work without any
> backing real swap. Though there is a future plan to move to zram
> eventually.

Interesting, if so why not just simply use zram?

>
> >
> > But it sounds like you would benefit from the zswap(ped) counters in
> > memory.stat at least.
>
> Yes and I think if we need zram specific counters/stats in future,
> those can be added then.
>
> >
> > Thanks, that is enlightening!
>