Messages in this thread | | | Date | Thu, 15 May 2014 11:45:31 +0900 | From | Heesub Shin <> | Subject | Re: [RFC PATCH 2/3] CMA: aggressively allocate the pages on cma reserved memory when not used |
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Hello,
On 05/15/2014 10:53 AM, Joonsoo Kim wrote: > On Tue, May 13, 2014 at 12:00:57PM +0900, Minchan Kim wrote: >> Hey Joonsoo, >> >> On Thu, May 08, 2014 at 09:32:23AM +0900, Joonsoo Kim wrote: >>> CMA is introduced to provide physically contiguous pages at runtime. >>> For this purpose, it reserves memory at boot time. Although it reserve >>> memory, this reserved memory can be used for movable memory allocation >>> request. This usecase is beneficial to the system that needs this CMA >>> reserved memory infrequently and it is one of main purpose of >>> introducing CMA. >>> >>> But, there is a problem in current implementation. The problem is that >>> it works like as just reserved memory approach. The pages on cma reserved >>> memory are hardly used for movable memory allocation. This is caused by >>> combination of allocation and reclaim policy. >>> >>> The pages on cma reserved memory are allocated if there is no movable >>> memory, that is, as fallback allocation. So the time this fallback >>> allocation is started is under heavy memory pressure. Although it is under >>> memory pressure, movable allocation easily succeed, since there would be >>> many pages on cma reserved memory. But this is not the case for unmovable >>> and reclaimable allocation, because they can't use the pages on cma >>> reserved memory. These allocations regard system's free memory as >>> (free pages - free cma pages) on watermark checking, that is, free >>> unmovable pages + free reclaimable pages + free movable pages. Because >>> we already exhausted movable pages, only free pages we have are unmovable >>> and reclaimable types and this would be really small amount. So watermark >>> checking would be failed. It will wake up kswapd to make enough free >>> memory for unmovable and reclaimable allocation and kswapd will do. >>> So before we fully utilize pages on cma reserved memory, kswapd start to >>> reclaim memory and try to make free memory over the high watermark. This >>> watermark checking by kswapd doesn't take care free cma pages so many >>> movable pages would be reclaimed. After then, we have a lot of movable >>> pages again, so fallback allocation doesn't happen again. To conclude, >>> amount of free memory on meminfo which includes free CMA pages is moving >>> around 512 MB if I reserve 512 MB memory for CMA. >>> >>> I found this problem on following experiment. >>> >>> 4 CPUs, 1024 MB, VIRTUAL MACHINE >>> make -j24 >>> >>> CMA reserve: 0 MB 512 MB >>> Elapsed-time: 234.8 361.8 >>> Average-MemFree: 283880 KB 530851 KB >>> >>> To solve this problem, I can think following 2 possible solutions. >>> 1. allocate the pages on cma reserved memory first, and if they are >>> exhausted, allocate movable pages. >>> 2. interleaved allocation: try to allocate specific amounts of memory >>> from cma reserved memory and then allocate from free movable memory. >> >> I love this idea but when I see the code, I don't like that. >> In allocation path, just try to allocate pages by round-robin so it's role >> of allocator. If one of migratetype is full, just pass mission to reclaimer >> with hint(ie, Hey reclaimer, it's non-movable allocation fail >> so there is pointless if you reclaim MIGRATE_CMA pages) so that >> reclaimer can filter it out during page scanning. >> We already have an tool to achieve it(ie, isolate_mode_t). > > Hello, > > I agree with leaving fast allocation path as simple as possible. > I will remove runtime computation for determining ratio in > __rmqueue_cma() and, instead, will use pre-computed value calculated > on the other path. > > I am not sure that whether your second suggestion(Hey relaimer part) > is good or not. In my quick thought, that could be helpful in the > situation that many free cma pages remained. But, it would be not helpful > when there are neither free movable and cma pages. In generally, most > workloads mainly uses movable pages for page cache or anonymous mapping. > Although reclaim is triggered by non-movable allocation failure, reclaimed > pages are used mostly by movable allocation. We can handle these allocation > request even if we reclaim the pages just in lru order. If we rotate > the lru list for finding movable pages, it could cause more useful > pages to be evicted. > > This is just my quick thought, so please let me correct if I am wrong.
We have an out of tree implementation that is completely the same with the approach Minchan said and it works, but it has definitely some side-effects as you pointed, distorting the LRU and evicting hot pages. I do not attach code fragments in this thread for some reasons, but it must be easy for yourself. I am wondering if it could help also in your case.
Thanks, Heesub
> >> >> And we couldn't do it in zone_watermark_ok with set/reset ALLOC_CMA? >> If possible, it would be better becauser it's generic function to check >> free pages and cause trigger reclaim/compaction logic. > > I guess, your *it* means ratio computation. Right? > I don't like putting it on zone_watermark_ok(). Although it need to > refer to free cma pages value which are also referred in zone_watermark_ok(), > this computation is for determining ratio, not for triggering > reclaim/compaction. And this zone_watermark_ok() is on more hot-path, so > putting this logic into zone_watermark_ok() looks not better to me. > > I will think better place to do it. > > Thanks. > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a> >
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