Re: [RFC PATCH 2/3] CMA: aggressively allocate the pages on cma reserved memory when not used

[Minchan Kim]

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.

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.

> 
> IMHO, in generally, memory shortage is made by movable allocation, so
> to distinguish allocation type and to handle them differently has
> marginal effect.

Again, please don't think workloads you know only and open the various
possiblity from the design although such consideration doesn't make code
ugly.

> 
> Anyway, I will think more deeply.

Yes, Please.

> 
> > 
> > > 
> > > > 
> > > > 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 meant just get_page_from_freelist like fair zone allocation for consistency
> > but as we discussed offline, i'm not against with you if it's not right place.
> 
> Okay :)
> 
> Thanks.
> 
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-- 
Kind regards,
Minchan Kim
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