On Thu, May 09, 2013 at 11:55:23AM +0100, Mel Gorman wrote:

On Thu, May 09, 2013 at 10:06:17AM +0400, Glauber Costa wrote:

quoted

[ Sending again, forgot to CC fsdevel. Shame on me ]
To Mel
======

I'm surprised Dave Chinner is not on the cc. He may or may not see it
on fsdevel.

I asked Glauber to send it to -fsdevel earlier today rather than
scattergun CC my RH email address with various patches in the
series. I'll be looking at this a bit more tomorrow now that I have
some new perf baselines I can use for comparison.

quoted

Mel, I have identified the overly aggressive behavior you noticed to be a bug
in the at-least-one-pass patch, that would ask the shrinkers to scan the full
batch even when total_scan < batch. They would do their best for it, and
eventually succeed. I also went further, and made that the behavior of direct
reclaim only - The only case that really matter for memcg, and one in which
we could argue that we are more or less desperate for small squeezes in memory.
Thank you very much for spotting this.

I haven't seen the relevant code yet but in general I do not think it is
a good idea for direct reclaim to potentially reclaim all of slabs like
this. Direct reclaim does not necessarily mean the system is desperate
for small amounts of memory. Lets take a few examples where it would be
a poor decision to reclaim all the slab pages within direct reclaim.

1. Direct reclaim triggers because kswapd is stalled writing pages for
   memcg (see code near comment "memcg doesn't have any dirty pages
   throttling"). A memcg dirtying its limit of pages may cause a lot of
   direct reclaim and dumping all the slab pages

2. Direct reclaim triggers because kswapd is writing pages out to swap.
   Similar to memcg above, kswapd failing to make forward progress triggers
   direct reclaim which then potentially reclaims all slab

3. Direct reclaim triggers because kswapd waits on congestion as there
   are too many pages under writeback. In this case, a large amounts of
   writes to slow storage like USB could result in all slab being reclaimed

4. The system has been up a long time, memory is fragmented and the page
   allocator enters direct reclaim/compaction to allocate THPs. It would
   be very unfortunate if allocating a THP reclaimed all the slabs

All that is potentially bad and likely to make Dave put in his cranky
pants. I would much prefer if direct reclaim and kswapd treated slab
similarly and not ask the shrinkers to do a full scan unless the alternative
is OOM kill.

Just keep in mind that I really don't care about micro-behaviours of
the shrinker algorithm. What I look at is the overall cache balance
under steady state workloads, the response to step changes in
workload and what sort of overhead is seen to maintain system
balance under memory pressure. So unless a micro-behaviour has an
impact at the macro level, I just don't care one way or the other.

But I can put on cranky panks if you want, Mel. :)

quoted

Running postmark on the final result (at least on my 2-node box) show something
a lot saner. We are still stealing more inodes than before, but by a factor of
around 15 %. Since the correct balance is somewhat heuristic anyway - I
personally think this is acceptable. But I am waiting to hear from you on this
matter. Meanwhile, I am investigating further to try to pinpoint where exactly
this comes from. It might either be because of the new node-aware behavior, or
because of the increased calculation precision in the first patch.

I'm going to defer to Dave as to whether that increased level of slab
reclaim is acceptable or not.

Depends on how it changes the balance of the system. I won't know
that until I run some new tests.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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