Re: The pagedaemon evicts ARC before scanning the inactive page list

From: Konstantin Belousov <kostikbel_at_gmail.com>
Date: Wed, 19 May 2021 06:24:58 +0300
On Tue, May 18, 2021 at 05:55:36PM -0600, Alan Somers wrote:
> On Tue, May 18, 2021 at 4:10 PM Mark Johnston <markj_at_freebsd.org> wrote:
> 
> > On Tue, May 18, 2021 at 04:00:14PM -0600, Alan Somers wrote:
> > > On Tue, May 18, 2021 at 3:45 PM Mark Johnston <markj_at_freebsd.org> wrote:
> > >
> > > > On Tue, May 18, 2021 at 03:07:44PM -0600, Alan Somers wrote:
> > > > > I'm using ZFS on servers with tons of RAM and running FreeBSD
> > > > > 12.2-RELEASE.  Sometimes they get into a pathological situation where
> > > > most
> > > > > of that RAM sits unused.  For example, right now one of them has:
> > > > >
> > > > > 2 GB   Active
> > > > > 529 GB Inactive
> > > > > 16 GB  Free
> > > > > 99 GB  ARC total
> > > > > 469 GB ARC max
> > > > > 86 GB  ARC target
> > > > >
> > > > > When a server gets into this situation, it stays there for days,
> > with the
> > > > > ARC target barely budging.  All that inactive memory never gets
> > reclaimed
> > > > > and put to a good use.  Frequently the server never recovers until a
> > > > reboot.
> > > > >
> > > > > I have a theory for what's going on.  Ever since r334508^ the
> > pagedaemon
> > > > > sends the vm_lowmem event _before_ it scans the inactive page list.
> > If
> > > > the
> > > > > ARC frees enough memory, then vm_pageout_scan_inactive won't need to
> > free
> > > > > any.  Is that order really correct?  For reference, here's the
> > relevant
> > > > > code, from vm_pageout_worker:
> > > >
> > > > That was the case even before r334508.  Note that prior to that
> > revision
> > > > vm_pageout_scan_inactive() would trigger vm_lowmem if pass > 0, before
> > > > scanning the inactive queue.  During a memory shortage we have pass >
> > 0.
> > > > pass == 0 only when the page daemon is scanning the active queue.
> > > >
> > > > > shortage = pidctrl_daemon(&vmd->vmd_pid, vmd->vmd_free_count);
> > > > > if (shortage > 0) {
> > > > >         ofree = vmd->vmd_free_count;
> > > > >         if (vm_pageout_lowmem() && vmd->vmd_free_count > ofree)
> > > > >                 shortage -= min(vmd->vmd_free_count - ofree,
> > > > >                     (u_int)shortage);
> > > > >         target_met = vm_pageout_scan_inactive(vmd, shortage,
> > > > >             &addl_shortage);
> > > > > } else
> > > > >         addl_shortage = 0
> > > > >
> > > > > Raising vfs.zfs.arc_min seems to workaround the problem.  But ideally
> > > > that
> > > > > wouldn't be necessary.
> > > >
> > > > vm_lowmem is too primitive: it doesn't tell subscribing subsystems
> > > > anything about the magnitude of the shortage.  At the same time, the VM
> > > > doesn't know much about how much memory they are consuming.  A better
> > > > strategy, at least for the ARC, would be reclaim memory based on the
> > > > relative memory consumption of each subsystem.  In your case, when the
> > > > page daemon goes to reclaim memory, it should use the inactive queue to
> > > > make up ~85% of the shortfall and reclaim the rest from the ARC.  Even
> > > > better would be if the ARC could use the page cache as a second-level
> > > > cache, like the buffer cache does.
> > > >
> > > > Today I believe the ARC treats vm_lowmem as a signal to shed some
> > > > arbitrary fraction of evictable data.  If the ARC is able to quickly
> > > > answer the question, "how much memory can I release if asked?", then
> > > > the page daemon could use that to determine how much of its reclamation
> > > > target should come from the ARC vs. the page cache.
> > > >
> > >
> > > I guess I don't understand why you would ever free from the ARC rather
> > than
> > > from the inactive list.  When is inactive memory ever useful?
> >
> > Pages in the inactive queue are either unmapped or haven't had their
> > mappings referenced recently.  But they may still be frequently accessed
> > by file I/O operations like sendfile(2).  That's not to say that
> > reclaiming from other subsystems first is always the right strategy, but
> > note also that the page daemon may scan the inactive queue many times in
> > between vm_lowmem calls.
> >
> 
> So By default ZFS tries to free (arc_target / 128) bytes of memory in
> arc_lowmem.  That's huge!  On this server, pidctrl_daemon typically
> requests 0-10MB, and arc_lowmem tries to free 600 MB.  It looks like it
> would be easy to modify vm_lowmem to include the total amount of memory
> that it wants freed.  I could make such a patch.  My next question is:
> what's the fastest way to generate a lot of inactive memory?  My first
> attempt was "find . | xargs md5", but that isn't terribly effective.  The
> production machines are doing a lot of "zfs recv" and running some busy Go
> programs, among other things, but I can't easily replicate that workload on

Is your machine ZFS-only?  If yes, then typical source of inactive memory
can be of two kinds:
- anonymous memory that apps allocate with facilities like malloc(3).
  If inactive is shrinkable then it is probably not, because dirty pages
  from anon objects must go through laundry->swap route to get evicted,
  and you did not mentioned swapping
- double-copy pages cached in v_objects of ZFS vnodes, clean or dirty.
  If unmapped, these are mostly a waste.  Even if mapped, the source
  of truth for data is ARC, AFAIU, so they can be dropped as well, since
  inactive state means that its content is not hot.

You can try to inspect the most outstanding objects adding to the
inactive queue with 'vmobject -o' to see where the most of inactive pages
come from.

If indeed they are double-copy, then perhaps ZFS can react even to the
current primitive vm_lowmem signal somewhat different. First, it could
do the pass over its vnodes and
- free clean unmapped pages 
- if some targets are not met after that, laundry dirty pages,
  then return to freeing clean unmapped pages
all that before ever touching its cache (ARC).
Received on Wed May 19 2021 - 03:24:58 UTC

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