Re: [PATCH] mm: fix draining remote pageset

[Michal Hocko <mhocko@suse.com>]

On Mon 21-08-23 16:30:18, Huang, Ying wrote:
> Michal Hocko <mhocko@suse.com> writes:
> 
> > On Wed 16-08-23 15:08:23, Huang, Ying wrote:
> >> Michal Hocko <mhocko@suse.com> writes:
> >> 
> >> > On Mon 14-08-23 09:59:51, Huang, Ying wrote:
> >> >> Hi, Michal,
> >> >> 
> >> >> Michal Hocko <mhocko@suse.com> writes:
> >> >> 
> >> >> > On Fri 11-08-23 17:08:19, Huang Ying wrote:
> >> >> >> If there is no memory allocation/freeing in the remote pageset after
> >> >> >> some time (3 seconds for now), the remote pageset will be drained to
> >> >> >> avoid memory wastage.
> >> >> >> 
> >> >> >> But in the current implementation, vmstat updater worker may not be
> >> >> >> re-queued when we are waiting for the timeout (pcp->expire != 0) if
> >> >> >> there are no vmstat changes, for example, when CPU goes idle.
> >> >> >
> >> >> > Why is that a problem?
> >> >> 
> >> >> The pages of the remote zone may be kept in the local per-CPU pageset
> >> >> for long time as long as there's no page allocation/freeing on the
> >> >> logical CPU.  In addition to the logical CPU goes idle, this is also
> >> >> possible if the logical CPU is busy in the user space.
> >> >
> >> > But why is this a problem? Is the scale of the problem sufficient to
> >> > trigger out of memory situations or be otherwise harmful?
> >> 
> >> This may trigger premature page reclaiming.  The pages in the PCP of the
> >> remote zone would have been freed to satisfy the page allocation for the
> >> remote zone to avoid page reclaiming.  It's highly possible that the
> >> local CPU just allocate/free from/to the remote zone temporarily.
> >
> > I am slightly confused here but I suspect by zone you mean remote pcp.
> > But more importantly is this a concern seen in real workload? Can you
> > quantify it in some manner? E.g. with this patch we have X more kswapd
> > scanning or even hit direct reclaim much less often.
> >> So,
> >> we should free PCP pages of the remote zone if there is no page
> >> allocation/freeing from/to the remote zone for 3 seconds.
> >
> > Well, I would argue this depends a lot. There are workloads which really
> > like to have CPUs idle and yet they would like to benefit from the
> > allocator fast path after that CPU goes out of idle because idling is
> > their power saving opportunity while workloads want to act quickly after
> > there is something to run.
> >
> > That being said, we really need some numbers (ideally from real world)
> > that proves this is not just a theoretical concern.
> 
> The behavior to drain the PCP of the remote zone (that is, remote PCP)
> was introduced in commit 4ae7c03943fc ("[PATCH] Periodically drain non
> local pagesets").  The goal of draining was well documented in the
> change log.  IIUC, some of your questions can be answered there?
> 
> This patch just restores the original behavior changed by commit
> 7cc36bbddde5 ("vmstat: on-demand vmstat workers V8").

Let me repeat. You need some numbers to show this is needed.
-- 
Michal Hocko
SUSE Labs