Messages in this thread | | | From | KOSAKI Motohiro <> | Subject | Re: [RFC][PATCH] mm: the page of MIGRATE_RESERVE don't insert into pcp | Date | Fri, 14 Nov 2008 13:31:01 +0900 (JST) |
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Hi
Sorry for late responce. Honestly, I have many bisiness trip in this month ;-)
> > > > > What your patch may help is the situation where the system is under intense > > > > > memory pressure, is dipping routinely into the lowmem reserves and mixing > > > > > with high-order atomic allocations. This seems a bit extreme. > > > > > > > > not so extreame. > > > > > > > > The linux page reclaim can't process in interrupt context. > > > > Sl network subsystem and driver often use MIGRATE_RESERVE memory although > > > > system have many reclaimable memory. > > > > > > > > > > Why are they often using MIGRATE_RESERVE, have you confirmed that? For that > > > to be happening, it implies that either memory is under intense pressure and > > > free pages are often below watermarks due to interrupt contexts or they are > > > frequently allocating high-order pages in interrupt context. Normal order-0 > > > allocations should be getting satisified from elsewhere as if the free page > > > counts are low, they would be direct reclaiming and that will likely be > > > outside of the MIGRATE_RESERVE areas. > > > > if inserting printk() in MIGRATE_RESERVE, I can observe MIGRATE_RESERVE > > page alloc easily although heavy workload don't run. > > but, there aren't my point. > > > > That's interesting. What is the size of a pageblock on your system and > is min_free_kbytes aligned to that value? If it's not aligned, it would > explain why MIGRATE_RESERVE pages are being used before the watermarks > are hit.
hmm, I don't have it yet. ok, I should investigate more.
> > ok, I guess my patch description was too poor (and a bit pointless). > > So, I retry it. > > > > (1) in general principal, the system should effort to avoid oom rather than > > performance if memory shortage happend. > > MIGRATE_RESERVE directly indicate memory shortage happend. > > and pcp caching can prevent another cpu allocation. > > MIGRATE_RESERVE does not directly indicate a memory shortage has > occured. Bear in mind that a number of pageblocks are marked > MIGRATE_RESERVE based on the value of the watermarks. In general, the > minimum number of pages kept free will be in the MIGRATE_RESERVE blocks > but it is not mandatory. > > > (2) MIGRATE_RESERVE is never searched by buffered_rmqueue() because > > allocflags_to_migratetype() never return MIGRATE_RESERVE. > > it doesn't work as cache. > > IOW, it don't help to increase performance. > > This is true. If MIGRATE_RESERVE pages are routinely being used and placed > on the pcp lists, the lists are not being used to their full potential > and your patch would make sense. > > > (3) if the system pass MIGRATE_RESERVE to free_hot_cold_page() continously, > > pcp queueing can reduce the number of grabing zone->lock. > > However, it is rate. because MIGRATE_RESERVE is emergency memory, > > Again, MIGRATE_RESERVE is not emergency memory. > > > and it is often used interupt context processing. > > continuous emergency memory allocation in interrupt context isn't so sane. > > > > Then, unqueueing MIGRATE_RESERVE page doesn't cause performance degression > > and, it can (a bit) increase realibility and I think merit is much over demerit. > > > > I'm now inclined to agree if you have shown that MIGRATE_RESERVE pages are > routinely ending up on the PCP lists.
Thanks!
So, now, I have two todo issue. - I should mesure performance. - I should investigate why MIGRATE_RESERVE is used on my machine.
I expect I finish to the end of next week.
> > Yup, I believe at that time your decision is right. > > However, I think the condision was changed (or to be able to change). > > > > (1) legacy pcp implementation deeply relate to struct zone size. > > and, to blow up struct zone size cause performance degression > > because cache miss increasing. > > However, it solved cristoph's cpu-alloc patch > > Indeed. > > > (2) legacy pcp doesn't have total number of pages restriction. > > So, increasing lists directly cause number of pages in pcp. > > it can cause oom problem on large numa environment. > > However, I think we can implement total number of pages restriction. > > > > Yes although knowing what the right size for each of the lists should be > so that the overall PCP lists are not huge is a tricky one.
Thank you for good advice.
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