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

[Heesub Shin]

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.
>
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