Messages in this thread | | | Date | Mon, 17 Jun 2013 16:54:09 +0900 | From | Heesub Shin <> | Subject | Re: [PATCH] mm: vmscan: remove redundant querying to shrinker |
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Hello,
On 06/17/2013 09:08 AM, Dave Chinner wrote: > On Fri, Jun 14, 2013 at 07:07:51PM +0900, Heesub Shin wrote: >> shrink_slab() queries each slab cache to get the number of >> elements in it. In most cases such queries are cheap but, >> on some caches. For example, Android low-memory-killer, >> which is operates as a slab shrinker, does relatively >> long calculation once invoked and it is quite expensive. > > As has already been pointed out, the low memory killer is a badly > broken piece of code. I can't run a normal machine with it enabled > because it randomly kills processes whenever memory pressure is > generated. What it does is simply broken and hence arguing that it > has too much overhead is not a convincing argument for changing core > shrinker infrastructure. > >> This patch removes redundant queries to shrinker function >> in the loop of shrink batch. >> >> Signed-off-by: Heesub Shin <heesub.shin@samsung.com> >> --- >> mm/vmscan.c | 4 ++-- >> 1 file changed, 2 insertions(+), 2 deletions(-) >> >> diff --git a/mm/vmscan.c b/mm/vmscan.c >> index fa6a853..11b6695 100644 >> --- a/mm/vmscan.c >> +++ b/mm/vmscan.c >> @@ -282,9 +282,8 @@ unsigned long shrink_slab(struct shrink_control *shrink, >> max_pass, delta, total_scan); >> >> while (total_scan >= batch_size) { >> - int nr_before; >> + int nr_before = max_pass; >> >> - nr_before = do_shrinker_shrink(shrinker, shrink, 0); >> shrink_ret = do_shrinker_shrink(shrinker, shrink, >> batch_size); >> if (shrink_ret == -1) >> @@ -293,6 +292,7 @@ unsigned long shrink_slab(struct shrink_control *shrink, >> ret += nr_before - shrink_ret; >> count_vm_events(SLABS_SCANNED, batch_size); >> total_scan -= batch_size; >> + max_pass = shrink_ret; >> >> cond_resched(); >> } > > Shrinkers run concurrently on different CPUs, and so the state of > the cache being shrunk can change significantly when cond_resched() > actually yields the CPU. Hence we need to recalculate the current > state of the cache before we shrink again to get an accurate idea of > how much work the current loop has done. If we get this badly wrong, > the caller of shrink_slab() will get an incorrect idea of how much > work was actually done by the shrinkers.... > > This problem is fixed in mmtom by the change of shrinker API that > results shrinker->scan_objects() returning the number of objects > freed directly, and hence it isn't necessary to have a > shrinker->count_objects() call in the scan loop anymore. i.e. the > reworked scan loop ends up like: > > while (total_scan >= batch_size) { > unsigned long ret; > shrinkctl->nr_to_scan = batch_size; > ret = shrinker->scan_objects(shrinker, shrinkctl); > > if (ret == SHRINK_STOP) > break; > freed += ret; > > count_vm_events(SLABS_SCANNED, batch_size); > total_scan -= batch_size; > } > > So we've already solved the problem you are concerned about.... > > Cheers, > > Dave. >
Thank you for all your comments. I have been keeping up with the mm-list for a while, but it was my first time having to send out patches and stuff. I only intended to ask for your reviews and feedbacks. Will make sure I get over the learning curve until next time around.
Thank you mm guys, Dave, Minchan and Andrew again.
-- Heesub
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