Re: [RFC PATCH 0/6] Do not call ->writepage[s] from direct reclaimand use a_ops->writepages() where possible

From: KAMEZAWA Hiroyuki
Date: Wed Jun 09 2010 - 21:34:31 EST

On Thu, 10 Jun 2010 02:10:35 +0100
Mel Gorman <mel@xxxxxxxxx> wrote:
> > # mount -t cgroup none /cgroups -o memory
> > # mkdir /cgroups/A
> > # echo $$ > /cgroups/A
> > # echo 300M > /cgroups/memory.limit_in_bytes
> > # make -j 8 or make -j 16
> >
> That sort of scenario would be barely pushed by kernbench. For a single
> kernel build, it's about 250-400M depending on the .config but it's still
> a bit unreliable. Critically, it's not the sort of workload that would have
> lots of long-lived mappings that would hurt a workload a lot if it was being
> paged out.

You're right. An excuse for me is that my concern is usually the amount of
swap-out and OOM at rapid/heavy pressure comes because it's visible to
users easily. So, I use short-lived process test.

> Maybe it would be reasonable as a starting point but we'd have to be
> very careful of the stack usage figures? I'm leaning towards this
> approach to start with.
> I'm preparing another release that takes my two most important patches
> about reclaim but also reduces usage in page relcaim (a combination of
> two previously released series). In combination, it might be ok for the
> memcg paths to reclaim pages from a stack perspective although the IO
> pattern might still blow.

sounds nice.

> > > I'm not sure how a flusher thread would work just within a cgroup. It
> > > would have to do a lot of searching to find the pages it needs
> > > considering that it's looking at inodes rather than pages.
> > >
> >
> > yes. So, I(we) need some way for coloring inode for selectable writeback.
> > But people in this area are very nervous about performance (me too ;), I've
> > not found the answer yet.
> >
> I worry that too much targetting of writing back a specific inode would
> have other consequences.

I personally think this(writeback scheduling) is a job for I/O cgroup.
So, I guess what memcg can do is dirty-ratio-limiting, at most. The user has to
set well-balanced combination of memory+I/O cgroup.
Sorry for wrong mixture of story.

> > Okay, I'll consider about how to kick kswapd via memcg or flusher-for-memcg.
> > Please go ahead as you want. I love good I/O pattern, too.
> >
> For the moment, I'm strongly leaning towards allowing memcg to write
> back pages. The IO pattern might not be great, but it would be in line
> with current behaviour. The critical question is really "is it possible
> to overflow the stack?".

Because I don't use XFS, I don't have relaiable answer, now. But, at least,
memcg's memory reclaim will never be called as top of do_select(), which
uses 1000 bytes.

We have to consider long-term fix for I/O patterns under memmcg but
please global-reclaim-update-first. We did in that way when splitting LRU
to ANON and FILE. I don't want to make memcg as a burden for updating
vmscan.c better.


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