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

From: Mark Johnston <markj_at_freebsd.org>
Date: Tue, 18 May 2021 18:10:42 -0400
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.
Received on Tue May 18 2021 - 22:10:42 UTC

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