Re: [PATCH] remove throttle_vm_writeout()

[Fengguang Wu]

On Fri, Oct 05, 2007 at 10:20:05AM -0700, Andrew Morton wrote:
> On Fri, 5 Oct 2007 20:30:28 +0800
> Fengguang Wu <wfg@xxxxxxxxxxxxxxxx> wrote:
> 
> > > commit c4e2d7ddde9693a4c05da7afd485db02c27a7a09
> > > Author: akpm <akpm>
> > > Date:   Sun Dec 22 01:07:33 2002 +0000
> > > 
> > >     [PATCH] Give kswapd writeback higher priority than pdflush
> > >     
> > >     The `low latency page reclaim' design works by preventing page
> > >     allocators from blocking on request queues (and by preventing them from
> > >     blocking against writeback of individual pages, but that is immaterial
> > >     here).
> > >     
> > >     This has a problem under some situations.  pdflush (or a write(2)
> > >     caller) could be saturating the queue with highmem pages.  This
> > >     prevents anyone from writing back ZONE_NORMAL pages.  We end up doing
> > >     enormous amounts of scenning.
> > 
> > Sorry, I cannot not understand it. We now have balanced aging between
> > zones. So the page allocations are expected to distribute proportionally
> > between ZONE_HIGHMEM and ZONE_NORMAL?
> 
> Sure, but we don't have one disk queue per disk per zone!  The queue is
> shared by all the zones.  So if writeback from one zone has filled the
> queue up, the kernel can't write back data from another zone.

Hmm, that's a problem. But I guess when one zone is full, other zones
will not be far away... It's a "sooner or later" problem.

> (Well, it can, by blocking in get_request_wait(), but that causes long and
> uncontrollable latencies).

I guess PF_SWAPWRITE processes still have good probability to stuck in
get_request_wait().  Because balance_dirty_pages() are allowed to
disregard the congestion.  It will be exhausting the available request
slots all the time.

Signed-off-by: Fengguang Wu <wfg@xxxxxxxxxxxxxxxx>
---
 mm/page-writeback.c |    1 +
 1 file changed, 1 insertion(+)

--- linux-2.6.23-rc8-mm2.orig/mm/page-writeback.c
+++ linux-2.6.23-rc8-mm2/mm/page-writeback.c
@@ -400,6 +400,7 @@ static void balance_dirty_pages(struct a
 			.sync_mode	= WB_SYNC_NONE,
 			.older_than_this = NULL,
 			.nr_to_write	= write_chunk,
+			.nonblocking	= 1,
 			.range_cyclic	= 1,
 		};
 

> > >     A test case is to mmap(MAP_SHARED) almost all of a 4G machine's memory,
> > >     then kill the mmapping applications.  The machine instantly goes from
> > >     0% of memory dirty to 95% or more.  pdflush kicks in and starts writing
> > >     the least-recently-dirtied pages, which are all highmem.  The queue is
> > >     congested so nobody will write back ZONE_NORMAL pages.  kswapd chews
> > >     50% of the CPU scanning past dirty ZONE_NORMAL pages and page reclaim
> > >     efficiency (pages_reclaimed/pages_scanned) falls to 2%.
> > >     
> > >     So this patch changes the policy for kswapd.  kswapd may use all of a
> > >     request queue, and is prepared to block on request queues.
> > >     
> > >     What will now happen in the above scenario is:
> > >     
> > >     1: The page alloctor scans some pages, fails to reclaim enough
> > >        memory and takes a nap in blk_congetion_wait().
> > >     
> > >     2: kswapd() will scan the ZONE_NORMAL LRU and will start writing
> > >        back pages.  (These pages will be rotated to the tail of the
> > >        inactive list at IO-completion interrupt time).
> > >     
> > >        This writeback will saturate the queue with ZONE_NORMAL pages.
> > >        Conveniently, pdflush will avoid the congested queues.  So we end up
> > >        writing the correct pages.
> > >     
> > >     In this test, kswapd CPU utilisation falls from 50% to 2%, page reclaim
> > >     efficiency rises from 2% to 40% and things are generally a lot happier.
> > 
> > We may see the same problem and improvement in the absent of 'all
> > writeback goes to one zone' assumption.
> > 
> > The problem could be:
> > - dirty_thresh is exceeded, so balance_dirty_pages() starts syncing
> >   data and quickly _congests_ the queue;
> 
> Or someone ran fsync(), or pdflush is writing back data because it exceeded
> dirty_writeback_centisecs, etc.

Ah, yes.

> > - dirty pages are slowly but continuously turned into clean pages by
> >   balance_dirty_pages(), but they still stay in the same place in LRU;
> > - the zones are mostly dirty/writeback pages, kswapd has a hard time
> >   finding the randomly distributed clean pages;
> > - kswapd cannot do the writeout because the queue is congested!
> > 
> > The improvement could be:
> > - kswapd is now explicitly preferred to do the writeout;
> > - the pages written by kswapd will be rotated and easy for kswapd to reclaim;
> > - it becomes possible for kswapd to wait for the congested queue,
> >   instead of doing the vmscan like mad.
> 
> Yeah.  In 2.4 and early 2.5, page-reclaim (both direct reclaim and kswapd,
> iirc) would throttle by waiting on writeout of a particular page.  This was
> a poor design, because writeback against a *particular* page can take
> anywhere from one millisecond to thirty seconds to complete, depending upon
> where the disk head is and all that stuff.
> 
> The critical change I made was to switch the throttling algorithm from
> "wait for one page to get written" to "wait for _any_ page to get written".
>  Becaue reclaim really doesn't care _which_ page got written: we want to
> wake up and start scanning again when _any_ page got written.

That must be a big improvement!

> That's what congestion_wait() does.
> 
> It is pretty crude.  It could be that writeback completed against pages which
> aren't in the correct zone, or it could be that some other task went and
> allocated the just-cleaned pages before this task can get running and
> reclaim them, or it could be that the just-written-back pages weren't
> reclaimable after all, etc.
> 
> It would take a mind-boggling amount of logic and locking to make all this
> 100% accurate and the need has never been demonstrated.  So page reclaim
> presently should be viewed as a polling algorithm, where the rate of
> polling is paced by the rate at which the IO system can retire writes.

Yeah. So the polling overheads are limited.

> > The congestion wait looks like a pretty natural way to throttle the kswapd.
> > Instead of doing the vmscan at 1000MB/s and actually freeing pages at
> > 60MB/s(about the write throughput), kswapd will be relaxed to do vmscan at
> > maybe 150MB/s.
> 
> Something like that.
> 
> The critical numbers to watch are /proc/vmstat's *scan* and *steal*.  Look:
> 
> akpm:/usr/src/25> uptime
>  10:08:14 up 10 days, 16:46, 15 users,  load average: 0.02, 0.05, 0.04
> akpm:/usr/src/25> grep steal /proc/vmstat
> pgsteal_dma 0
> pgsteal_dma32 0
> pgsteal_normal 0
> pgsteal_high 0
> pginodesteal 0
> kswapd_steal 1218698
> kswapd_inodesteal 266847
> akpm:/usr/src/25> grep scan /proc/vmstat      
> pgscan_kswapd_dma 0
> pgscan_kswapd_dma32 1246816
> pgscan_kswapd_normal 0
> pgscan_kswapd_high 0
> pgscan_direct_dma 0
> pgscan_direct_dma32 448
> pgscan_direct_normal 0
> pgscan_direct_high 0
> slabs_scanned 2881664
> 
> Ignore kswapd_inodesteal and slabs_scanned.  We see that this machine has
> scanned 1246816+448 pages and has reclaimed (stolen) 1218698 pages.  That's
> a reclaim success rate of 97.7%, which is pretty damn good - this machine
> is just a lightly-loaded 3GB desktop.
> 
> When testing reclaim, it is critical that this ratio be monitored (vmmon.c
> from ext3-tools is a vmstat-like interface to /proc/vmstat).  If the
> reclaim efficiency falls below, umm, 25% then things are getting into some
> trouble.

Nice tool!
I've been watching the raw numbers by writing scripts. This one can be
written as:

#!/bin/bash

while true; do
	while read a b
	do
		eval $a=$b
	done < /proc/vmstat

	uptime=$(</proc/uptime)

	scan=$((
		pgscan_kswapd_dma +
		pgscan_kswapd_dma32 +
		pgscan_kswapd_normal +
		pgscan_kswapd_high +
		pgscan_direct_dma +
		pgscan_direct_dma32 +
		pgscan_direct_normal +
		pgscan_direct_high
		))

	steal=$((
		pgsteal_dma +
		pgsteal_dma32 +
		pgsteal_normal +
		pgsteal_high
		))

	ratio=$((100*steal/(scan+1)))

	echo -e "$uptime\t$ratio%\t$steal\t$scan"
        sleep 1;
done

Not surprisingly, I see nice numbers on my desktop:

9517.99 9368.60 96%     1898452 1961536

> Actually, 25% is still pretty good.  We scan 4 pages for each reclaimed
> page, but the amount of wall time which that takes is vastly less than the
> time to write one page, bearing in mind that these things tend to be seeky
> as hell.  But still, keeping an eye on the reclaim efficiency is just your
> basic starting point for working on page reclaim.

Thank you for the nice tip :-)

Fengguang

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