On Wed, May 14, 2014 at 02:12:19PM +0530, Aneesh Kumar K.V wrote: > Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> writes: > > > 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. > > > Another issue i am facing with the current code is the atomic allocation > failing even with large number of CMA pages around. In my case we never > reclaimed because large part of the memory is consumed by the page cache and > for that, free memory check doesn't include at free_cma. I will test > with this patchset and update here once i have the results. > Hello, Could you elaborate more on your issue? I can't completely understand your problem. So your atomic allocation is movable? And although there are many free cma pages, that request is fail? > > > > 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 tested #1 approach and found the problem. Although free memory on > > meminfo can move around low watermark, there is large fluctuation on free > > memory, because too many pages are reclaimed when kswapd is invoked. > > Reason for this behaviour is that successive allocated CMA pages are > > on the LRU list in that order and kswapd reclaim them in same order. > > These memory doesn't help watermark checking from kwapd, so too many > > pages are reclaimed, I guess. > > > > So, I implement #2 approach. > > One thing I should note is that we should not change allocation target > > (movable list or cma) on each allocation attempt, since this prevent > > allocated pages to be in physically succession, so some I/O devices can > > be hurt their performance. To solve this, I keep allocation target > > in at least pageblock_nr_pages attempts and make this number reflect > > ratio, free pages without free cma pages to free cma pages. With this > > approach, system works very smoothly and fully utilize the pages on > > cma reserved memory. > > > > Following is the experimental result of this patch. > > > > 4 CPUs, 1024 MB, VIRTUAL MACHINE > > make -j24 > > > > <Before> > > CMA reserve: 0 MB 512 MB > > Elapsed-time: 234.8 361.8 > > Average-MemFree: 283880 KB 530851 KB > > pswpin: 7 110064 > > pswpout: 452 767502 > > > > <After> > > CMA reserve: 0 MB 512 MB > > Elapsed-time: 234.2 235.6 > > Average-MemFree: 281651 KB 290227 KB > > pswpin: 8 8 > > pswpout: 430 510 > > > > There is no difference if we don't have cma reserved memory (0 MB case). > > But, with cma reserved memory (512 MB case), we fully utilize these > > reserved memory through this patch and the system behaves like as > > it doesn't reserve any memory. > > > > With this patch, we aggressively allocate the pages on cma reserved memory > > so latency of CMA can arise. Below is the experimental result about > > latency. > > > > 4 CPUs, 1024 MB, VIRTUAL MACHINE > > CMA reserve: 512 MB > > Backgound Workload: make -jN > > Real Workload: 8 MB CMA allocation/free 20 times with 5 sec interval > > > > N: 1 4 8 16 > > Elapsed-time(Before): 4309.75 9511.09 12276.1 77103.5 > > Elapsed-time(After): 5391.69 16114.1 19380.3 34879.2 > > > > So generally we can see latency increase. Ratio of this increase > > is rather big - up to 70%. But, under the heavy workload, it shows > > latency decrease - up to 55%. This may be worst-case scenario, but > > reducing it would be important for some system, so, I can say that > > this patch have advantages and disadvantages in terms of latency. > > > > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> > > > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > > index fac5509..3ff24d4 100644 > > --- a/include/linux/mmzone.h > > +++ b/include/linux/mmzone.h > > @@ -389,6 +389,12 @@ struct zone { > > int compact_order_failed; > > #endif > > > > +#ifdef CONFIG_CMA > > + int has_cma; > > + int nr_try_cma; > > + int nr_try_movable; > > +#endif > > > Can you write documentation around this ? > Okay. > > + > > ZONE_PADDING(_pad1_) > > > > /* Fields commonly accessed by the page reclaim scanner */ > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > index 674ade7..6f2b27b 100644 > > --- a/mm/page_alloc.c > > +++ b/mm/page_alloc.c > > @@ -788,6 +788,16 @@ void __init __free_pages_bootmem(struct page *page, unsigned int order) > > } > > > > #ifdef CONFIG_CMA > > +void __init init_alloc_ratio_counter(struct zone *zone) > > +{ > > + if (zone->has_cma) > > + return; > > + > > + zone->has_cma = 1; > > + zone->nr_try_movable = 0; > > + zone->nr_try_cma = 0; > > +} > > + > > /* Free whole pageblock and set its migration type to MIGRATE_CMA. */ > > void __init init_cma_reserved_pageblock(struct page *page) > > { > > @@ -803,6 +813,7 @@ void __init init_cma_reserved_pageblock(struct page *page) > > set_pageblock_migratetype(page, MIGRATE_CMA); > > __free_pages(page, pageblock_order); > > adjust_managed_page_count(page, pageblock_nr_pages); > > + init_alloc_ratio_counter(page_zone(page)); > > } > > #endif > > > > @@ -1136,6 +1147,69 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype) > > return NULL; > > } > > > > +#ifdef CONFIG_CMA > > +static struct page *__rmqueue_cma(struct zone *zone, unsigned int order, > > + int migratetype) > > +{ > > + long free, free_cma, free_wmark; > > + struct page *page; > > + > > + if (migratetype != MIGRATE_MOVABLE || !zone->has_cma) > > + return NULL; > > + > > + if (zone->nr_try_movable) > > + goto alloc_movable; > > + > > +alloc_cma: > > + if (zone->nr_try_cma) { > > + /* Okay. Now, we can try to allocate the page from cma region */ > > + zone->nr_try_cma--; > > + page = __rmqueue_smallest(zone, order, MIGRATE_CMA); > > + > > + /* CMA pages can vanish through CMA allocation */ > > + if (unlikely(!page && order == 0)) > > + zone->nr_try_cma = 0; > > + > > + return page; > > + } > > + > > + /* Reset ratio counter */ > > + free_cma = zone_page_state(zone, NR_FREE_CMA_PAGES); > > + > > + /* No cma free pages, so recharge only movable allocation */ > > + if (free_cma <= 0) { > > + zone->nr_try_movable = pageblock_nr_pages; > > + goto alloc_movable; > > + } > > + > > + free = zone_page_state(zone, NR_FREE_PAGES); > > + free_wmark = free - free_cma - high_wmark_pages(zone); > > + > > + /* > > + * free_wmark is below than 0, and it means that normal pages > > + * are under the pressure, so we recharge only cma allocation. > > + */ > > + if (free_wmark <= 0) { > > + zone->nr_try_cma = pageblock_nr_pages; > > + goto alloc_cma; > > + } > > + > > + if (free_wmark > free_cma) { > > + zone->nr_try_movable = > > + (free_wmark * pageblock_nr_pages) / free_cma; > > + zone->nr_try_cma = pageblock_nr_pages; > > + } else { > > + zone->nr_try_movable = pageblock_nr_pages; > > + zone->nr_try_cma = free_cma * pageblock_nr_pages / free_wmark; > > + } > > Can you add the commit message documentation here. > Okay. > > + > > + /* Reset complete, start on movable first */ > > +alloc_movable: > > + zone->nr_try_movable--; > > + return NULL; > > +} > > +#endif > > + > > /* > > * Do the hard work of removing an element from the buddy allocator. > > * Call me with the zone->lock already held. > > @@ -1143,10 +1217,14 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype) > > static struct page *__rmqueue(struct zone *zone, unsigned int order, > > int migratetype) > > { > > - struct page *page; > > + struct page *page = NULL; > > + > > + if (IS_ENABLED(CONFIG_CMA)) > > + page = __rmqueue_cma(zone, order, migratetype); > > It would be better to move the migrate check here, So that it becomes > > /* For migrate movable allocation try cma area first */ > if (IS_ENABLED(CONFIG_CMA) && (migratetype == MIGRATE_MOVABLE)) > > Okay. But it makes no difference between current code and your suggestion, because __rmqueue_cma would be inlined by compiler optimization. Thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>