Re: [PATCH 0/4] (CMA_AGGRESSIVE) Make CMA memory be more aggressive about allocation

[Peter Hurley]

On 10/16/2014 04:55 AM, Laura Abbott wrote:
> On 10/15/2014 8:35 PM, Hui Zhu wrote:
>> In fallbacks of page_alloc.c, MIGRATE_CMA is the fallback of
>> MIGRATE_MOVABLE.
>> MIGRATE_MOVABLE will use MIGRATE_CMA when it doesn't have a page in
>> order that Linux kernel want.
>>
>> If a system that has a lot of user space program is running, for
>> instance, an Android board, most of memory is in MIGRATE_MOVABLE and
>> allocated.  Before function __rmqueue_fallback get memory from
>> MIGRATE_CMA, the oom_killer will kill a task to release memory when
>> kernel want get MIGRATE_UNMOVABLE memory because fallbacks of
>> MIGRATE_UNMOVABLE are MIGRATE_RECLAIMABLE and MIGRATE_MOVABLE.
>> This status is odd.  The MIGRATE_CMA has a lot free memory but Linux
>> kernel kill some tasks to release memory.
>>
>> This patch series adds a new function CMA_AGGRESSIVE to make CMA memory
>> be more aggressive about allocation.
>> If function CMA_AGGRESSIVE is available, when Linux kernel call function
>> __rmqueue try to get pages from MIGRATE_MOVABLE and conditions allow,
>> MIGRATE_CMA will be allocated as MIGRATE_MOVABLE first.  If MIGRATE_CMA
>> doesn't have enough pages for allocation, go back to allocate memory from
>> MIGRATE_MOVABLE.
>> Then the memory of MIGRATE_MOVABLE can be kept for MIGRATE_UNMOVABLE and
>> MIGRATE_RECLAIMABLE which doesn't have fallback MIGRATE_CMA.
>>
> 
> It's good to see another proposal to fix CMA utilization. Do you have
> any data about the success rate of CMA contiguous allocation after
> this patch series? I played around with a similar approach of using
> CMA for MIGRATE_MOVABLE allocations and found that although utilization
> did increase, contiguous allocations failed at a higher rate and were
> much slower. I see what this series is trying to do with avoiding
> allocation from CMA pages when a contiguous allocation is progress.
> My concern is that there would still be problems with contiguous
> allocation after all the MIGRATE_MOVABLE fallback has happened.

What impact does this series have on x86 platforms now that CMA is the
backup allocator for all iommu dma allocations?

Regards,
Peter Hurley
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