Messages in this thread | | | From | KOSAKI Motohiro <> | Subject | Re: [RFC] high system time & lock contention running large mixed workload | Date | Tue, 1 Dec 2009 21:23:23 +0900 (JST) |
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(cc to some related person)
> The cause was determined to be the unconditional call to > page_referenced() for every mapped page encountered in > shrink_active_list(). page_referenced() takes the anon_vma->lock and > calls page_referenced_one() for each vma. page_referenced_one() then > calls page_check_address() which takes the pte_lockptr spinlock. If > several CPUs are doing this at the same time there is a lot of > pte_lockptr spinlock contention with the anon_vma->lock held. This > causes contention on the anon_vma->lock, stalling in the fo and very > high system time. > > Before the splitLRU patch shrink_active_list() would only call > page_referenced() when reclaim_mapped got set. reclaim_mapped only got > set when the priority worked its way from 12 all the way to 7. This > prevented page_referenced() from being called from shrink_active_list() > until the system was really struggling to reclaim memory. > > On way to prevent this is to change page_check_address() to execute a > spin_trylock(ptl) when it was called by shrink_active_list() and simply > fail if it could not get the pte_lockptr spinlock. This will make > shrink_active_list() consider the page not referenced and allow the > anon_vma->lock to be dropped much quicker. > > The attached patch does just that, thoughts???
At first look,
- We have to fix this issue certenally. - But your patch is a bit risky.
Your patch treat trylock(pte-lock) failure as no accessced. but generally lock contention imply to have contention peer. iow, the page have reference bit typically. then, next shrink_inactive_list() move it active list again. that's suboptimal result.
However, we can't treat lock-contention as page-is-referenced simply. if it does, the system easily go into OOM.
So, if (priority < DEF_PRIORITY - 2) page_referenced() else page_refenced_trylock()
is better? On typical workload, almost vmscan only use DEF_PRIORITY. then, if priority==DEF_PRIORITY situation don't cause heavy lock contention, the system don't need to mind the contention. anyway we can't avoid contention if the system have heavy memory pressure.
btw, current shrink_active_list() have unnecessary page_mapping_inuse() call. it prevent to drop page reference bit from unmapped cache page. it mean we protect unmapped cache page than mapped page. it is strange.
Unfortunately, I don't have enough development time today. I'll working on tommorow.
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