Re: [PATCH 8/8] memcg asyncrhouns reclaim workqueue

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Fri, 20 May 2011 18:26:40 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Sat, 21 May 2011 09:41:50 +0900 Hiroyuki Kamezawa <kamezawa.hiroyuki@gmail.com> wrote:
> 
> > 2011/5/21 Andrew Morton <akpm@linux-foundation.org>:
> > > On Fri, 20 May 2011 12:48:37 +0900
> > > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > >
> > >> workqueue for memory cgroup asynchronous memory shrinker.
> > >>
> > >> This patch implements the workqueue of async shrinker routine. each
> > >> memcg has a work and only one work can be scheduled at the same time.
> > >>
> > >> If shrinking memory doesn't goes well, delay will be added to the work.
> > >>
> > >
> > > When this code explodes (as it surely will), users will see large
> > > amounts of CPU consumption in the work queue thread. __We want to make
> > > this as easy to debug as possible, so we should try to make the
> > > workqueue's names mappable back onto their memcg's. __And anything else
> > > we can think of to help?
> > >
> > 
> > I had a patch for showing per-memcg reclaim latency stats. It will be help.
> > I'll add it again to this set. I just dropped it because there are many patches
> > onto memory.stat in flight..
> 
> Will that patch help us when users report the memcg equivalent of
> "kswapd uses 99% of CPU"?
> 
I think so. Each memcg shows what amount of cpu is used.

But, maybe it's not an easy interface. I have several idea.


An idea I have is to rename task->comm by overwrite from  kworker/u:%d as
to memcg/%d when the work is scheduled. I think this can be implemented in very
simple interface and flags to workqueue. Then, ps -elf can show what was goin on.
If necessary, I'll add a hardlimit of cpu usage for a work or I'll limit
the number of thread for memcg workqueue. 

Considering there are user who uses 2000+ memcg on a system, a thread per a memcg
was not a choice to me. Another idea was thread poll or workqueue. Because thread
pool can be a poor reimplemenation of workqueue, I used workqueue.

I'll implement some idea in above to the next version. 


> > >
> > >> + __ __ limit = res_counter_read_u64(&mem->res, RES_LIMIT);
> > >> + __ __ shrink_to = limit - MEMCG_ASYNC_MARGIN - PAGE_SIZE;
> > >> + __ __ usage = res_counter_read_u64(&mem->res, RES_USAGE);
> > >> + __ __ if (shrink_to <= usage) {
> > >> + __ __ __ __ __ __ required = usage - shrink_to;
> > >> + __ __ __ __ __ __ required = (required >> PAGE_SHIFT) + 1;
> > >> + __ __ __ __ __ __ /*
> > >> + __ __ __ __ __ __ __* This scans some number of pages and returns that memory
> > >> + __ __ __ __ __ __ __* reclaim was slow or now. If slow, we add a delay as
> > >> + __ __ __ __ __ __ __* congestion_wait() in vmscan.c
> > >> + __ __ __ __ __ __ __*/
> > >> + __ __ __ __ __ __ congested = mem_cgroup_shrink_static_scan(mem, (long)required);
> > >> + __ __ }
> > >> + __ __ if (test_bit(ASYNC_NORESCHED, &mem->async_flags)
> > >> + __ __ __ __ || mem_cgroup_async_should_stop(mem))
> > >> + __ __ __ __ __ __ goto finish_scan;
> > >> + __ __ /* If memory reclaim couldn't go well, add delay */
> > >> + __ __ if (congested)
> > >> + __ __ __ __ __ __ delay = HZ/10;
> > >
> > > Another magic number.
> > >
> > > If Moore's law holds, we need to reduce this number by 1.4 each year.
> > > Is this good?
> > >
> > 
> > not good.  I just used the same magic number now used with wait_iff_congested.
> > Other than timer, I can use pagein/pageout event counter. If we have
> > dirty_ratio,
> > I may able to link this to dirty_ratio and wait until dirty_ratio is enough low.
> > Or, wake up again hit limit.
> > 
> > Do you have suggestion ?
> > 
> 
> mm..  It would be pretty easy to generate an estimate of "pages scanned
> per second" from the contents of (and changes in) the scan_control. 

Hmm.

> Konwing that datum and knowing the number of pages in the memcg, we
> should be able to come up with a delay period which scales
> appropriately with CPU speed and with memory size?
> 
> Such a thing could be used to rationalise magic delays in other places,
> hopefully.
> 

Ok, I'll conder that. Thank you for nice idea.


> > 
> > >> + __ __ queue_delayed_work(memcg_async_shrinker, &mem->async_work, delay);
> > >> + __ __ return;
> > >> +finish_scan:
> > >> + __ __ cgroup_release_and_wakeup_rmdir(&mem->css);
> > >> + __ __ clear_bit(ASYNC_RUNNING, &mem->async_flags);
> > >> + __ __ return;
> > >> +}
> > >> +
> > >> +static void run_mem_cgroup_async_shrinker(struct mem_cgroup *mem)
> > >> +{
> > >> + __ __ if (test_bit(ASYNC_NORESCHED, &mem->async_flags))
> > >> + __ __ __ __ __ __ return;
> > >
> > > I can't work out what ASYNC_NORESCHED does. __Is its name well-chosen?
> > >
> > how about BLOCK/STOP_ASYNC_RECLAIM ?
> 
> I can't say - I don't know what it does!  Or maybe I did, and immediately
> forgot ;)
> 

I'll find a better name ;)

Thanks,
-Kame

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