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).
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 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
> +
> 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;
> + }
> +
> + /* 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);
>
> retry_reserve:
> - page = __rmqueue_smallest(zone, order, migratetype);
> + if (!page)
> + page = __rmqueue_smallest(zone, order, migratetype);
>
> if (unlikely(!page) && migratetype != MIGRATE_RESERVE) {
> page = __rmqueue_fallback(zone, order, migratetype);
> @@ -4849,6 +4927,8 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
> zone_seqlock_init(zone);
> zone->zone_pgdat = pgdat;
> zone_pcp_init(zone);
> + if (IS_ENABLED(CONFIG_CMA))
> + zone->has_cma = 0;
>
> /* For bootup, initialized properly in watermark setup */
> mod_zone_page_state(zone, NR_ALLOC_BATCH, zone->managed_pages);
> --
> 1.7.9.5
>
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--
Kind regards,
Minchan Kim
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