On Mon, May 19, 2014 at 11:53:05AM +0900, Minchan Kim wrote: > On Mon, May 19, 2014 at 11:11:21AM +0900, Joonsoo Kim wrote: > > On Thu, May 15, 2014 at 11:43:53AM +0900, Minchan Kim wrote: > > > On Thu, May 15, 2014 at 10:53:01AM +0900, 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. > > > > > > Sounds good. > > > > > > > > > > > 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. > > > > > > Why should reclaimer reclaim unnecessary pages? > > > So, your answer is that it would be better because upcoming newly allocated > > > pages would be allocated easily without interrupt. But it could reclaim > > > too much pages until watermark for unmovable allocation is okay. > > > Even, sometime, you might see OOM. > > > > > > Moreover, how could you handle current trobule? > > > For example, there is atomic allocation and the only thing to save the world > > > is kswapd because it's one of kswapd role but kswapd is spending many time to > > > reclaim CMA pages, which is pointless so the allocation would be easily failed. > > > > Hello, > > > > I guess that it isn't the problem. In lru, movable pages and cma pages > > would be interleaved. So it doesn't takes too long time to get the > > page for non-movable allocation. > > Please, don't assume there are ideal LRU ordering. > Newly allocated page by fairness allocation is located by head of LRU > while old pages are approaching the tail so there is huge time gab. > During the time, old pages could be dropped/promoting so one of side > could be filled with one type rather than interleaving both types pages > you expected. I assumed general case, not ideal case. Your example can be possible, but would be corner case. > > Additionally, if you uses syncable backed device like ramdisk/zram > or something, pageout can be synchronized with page I/O. > In this case, reclaim time wouldn't be trivial than async I/O. > For exmaple, zram-swap case, it needs page copy + comperssion and > the speed depends on your CPU speed. This is a general problem what zram-swap have, although reclaiming cma pages worse the situation. 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