Re: [PATCH] RFC: vmscan: add min_filelist_kbytes sysctl for protecting the working set

[Mandeep Singh Baines <msb@chromium.org>]

Rik van Riel (riel@redhat.com) wrote:
> On 11/03/2010 06:40 PM, Mandeep Singh Baines wrote:
> 
> >I've created a patch which takes a slightly different approach.
> >Instead of limiting how fast pages get reclaimed, the patch limits
> >how fast the active list gets scanned. This should result in the
> >active list being a better measure of the working set. I've seen
> >fairly good results with this patch and a scan inteval of 1
> >centisecond. I see no thrashing when the scan interval is non-zero.
> >
> >I've made it a tunable because I don't know what to set the scan
> >interval. The final patch could set the value based on HZ and some
> >other system parameters. Maybe relate it to sched_period?
> 
> I like your approach. For file pages it looks like it
> could work fine, since new pages always start on the
> inactive file list.
> 
> However, for anonymous pages I could see your patch
> leading to problems, because all anonymous pages start
> on the active list.  With a scan interval of 1
> centiseconds, that means there would be a limit of 3200
> pages, or 12MB of anonymous memory that can be moved to
> the inactive list a second.
> 

Good point.

> I have seen systems with single SATA disks push out
> several times that to swap per second, which matters
> when someone starts up a program that is just too big
> to fit in memory and requires that something is pushed
> out.
> 
> That would reduce the size of the inactive list to
> zero, reducing our page replacement to a slow FIFO
> at best, causing false OOM kills at worst.
> 
> Staying with a default of 0 would of course not do
> anything, which would make merging the code not too
> useful.
> 
> I believe we absolutely need to preserve the ability
> to evict pages quickly, when new pages are brought
> into memory or allocated quickly.
> 

Agree.

Instead of doing one scan of SWAP_CLUSTER_MAX pages per vmscan_interval,
we could one "full" scan per vmscan_interval. You could do one full scan
all at once or scan SWAP_CLUSTER_MAX every scan until you've scanned
the whole list.

Psuedo code:

if (zone->to_scan[file] == 0 && !list_scanned_recently(zone, file))
	zone->to_scan[file] = list_get_size(zone, file);
if (zone->to_scan[file]) {
	shrink_active_list(nr_to_scan, zone, sc, priority, file);
	zone->to_scan[file] -= min(zone->to_scan[file], nr_to_scan);
}

> However, speed limits are probably a very good idea
> once a cache has been reduced to a smaller size, or
> when most IO bypasses the reclaim-speed-limited cache.
> 
> -- 
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