Messages in this thread | | | Subject | Re: [RESEND PATCH 1/2 -mm] mm: account lazy free pages separately | From | Yang Shi <> | Date | Wed, 14 Aug 2019 21:51:47 -0700 |
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On 8/14/19 4:08 AM, Michal Hocko wrote: > On Mon 12-08-19 10:00:17, Yang Shi wrote: >> >> On 8/12/19 2:34 AM, Michal Hocko wrote: >>> On Fri 09-08-19 16:54:43, Yang Shi wrote: >>>> On 8/9/19 11:26 AM, Yang Shi wrote: >>>>> On 8/9/19 11:02 AM, Michal Hocko wrote: >>> [...] >>>>>> I have to study the code some more but is there any reason why those >>>>>> pages are not accounted as proper THPs anymore? Sure they are partially >>>>>> unmaped but they are still THPs so why cannot we keep them accounted >>>>>> like that. Having a new counter to reflect that sounds like papering >>>>>> over the problem to me. But as I've said I might be missing something >>>>>> important here. >>>>> I think we could keep those pages accounted for NR_ANON_THPS since they >>>>> are still THP although they are unmapped as you mentioned if we just >>>>> want to fix the improper accounting. >>>> By double checking what NR_ANON_THPS really means, >>>> Documentation/filesystems/proc.txt says "Non-file backed huge pages mapped >>>> into userspace page tables". Then it makes some sense to dec NR_ANON_THPS >>>> when removing rmap even though they are still THPs. >>>> >>>> I don't think we would like to change the definition, if so a new counter >>>> may make more sense. >>> Yes, changing NR_ANON_THPS semantic sounds like a bad idea. Let >>> me try whether I understand the problem. So we have some THP in >>> limbo waiting for them to be split and unmapped parts to be freed, >>> right? I can see that page_remove_anon_compound_rmap does correctly >>> decrement NR_ANON_MAPPED for sub pages that are no longer mapped by >>> anybody. LRU pages seem to be accounted properly as well. As you've >>> said NR_ANON_THPS reflects the number of THPs mapped and that should be >>> reflecting the reality already IIUC. >>> >>> So the only problem seems to be that deferred THP might aggregate a lot >>> of immediately freeable memory (if none of the subpages are mapped) and >>> that can confuse MemAvailable because it doesn't know about the fact. >>> Has an skewed counter resulted in a user observable behavior/failures? >> No. But the skewed counter may make big difference for a big scale cluster. >> The MemAvailable is an important factor for cluster scheduler to determine >> the capacity. > But MemAvailable is a very rough estimation. Is relying on it really a > good measure? I mean there is a lot of reclaimable memory that is not > reflected there (some fs. internal data structures, networking buffers > etc.)
Yes, I agree there are other freeable objects not accounted into MemAvailable. Their size depends on the workload. But, deferred split THPs seems more common with the common workloads. A simple run with MariaDB test of mmtest shows it could generate over fifteen thousand deferred split THPs (accumulated around 30G in one hour run, 75% of 40G memory for my VM). So, it may be worth accounting deferred split THPs in MemAvailable.
> > [...] > >>> accounting the full THP correct? What if subpages are still mapped? >> "Deferred split" definitely doesn't mean they are free. When memory pressure >> is hit, they would be split, then the unmapped normal pages would be freed. >> So, when calculating MemAvailable, they are not accounted 100%, but like >> "available += lazyfree - min(lazyfree / 2, wmark_low)", just like how page >> cache is accounted. > Then this is even more dubious IMHO. > >> We could get more accurate account, i.e. checking each sub page's mapcount >> when accounting, but it may change before shrinker start scanning. So, just >> use the ballpark estimation to trade off the complexity for accurate >> accounting. > I do not see much point in fixing up one particular counter when there > is a whole lot that is even not considered. I would rather live with the > fact that MemAvailable is only very rough estimate then whack a mole on > any memory consumer that is freeable directly or indirectly via memory > reclaim. Because this is likely to be always subtly broken and only > visible under very specific workloads so there is no way to test for it.
I saw Vlastimil suggested KReclaimable, it seems a good fit. If so we don't need create a new counter anymore.
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