Re: [RFC][PATCH 03/26] mm, mpol: add MPOL_MF_LAZY ...

Rik van Riel
2012-07-06T19:13:43Z

On 03/23/2012 07:50 AM, Mel Gorman wrote:
> On Fri, Mar 16, 2012 at 03:40:31PM +0100, Peter Zijlstra wrote:
>> From: Lee Schermerhorn<Lee.Schermerhorn@hp.com>
>>
>> This patch adds another mbind() flag to request "lazy migration".
>> The flag, MPOL_MF_LAZY, modifies MPOL_MF_MOVE* such that the selected
>> pages are simply unmapped from the calling task's page table ['_MOVE]
>> or from all referencing page tables [_MOVE_ALL]. Anon pages will first
>> be added to the swap [or migration?] cache, if necessary. The pages
>> will be migrated in the fault path on "first touch", if the policy
>> dictates at that time.
>>
>> <SNIP>
>>
>> @@ -950,6 +950,98 @@ static int unmap_and_move_huge_page(new_
>> }
>>
>> /*
>> + * Lazy migration: just unmap pages, moving anon pages to swap cache, if
>> + * necessary. Migration will occur, if policy dictates, when a task faults
>> + * an unmapped page back into its page table--i.e., on "first touch" after
>> + * unmapping. Note that migrate-on-fault only migrates pages whose mapping
>> + * [e.g., file system] supplies a migratepage op, so we skip pages that
>> + * wouldn't migrate on fault.
>> + *
>> + * Pages are placed back on the lru whether or not they were successfully
>> + * unmapped. Like migrate_pages().
>> + *
>> + * Unline migrate_pages(), this function is only called in the context of
>> + * a task that is unmapping it's own pages while holding its map semaphore
>> + * for write.
>> + */
>> +int migrate_pages_unmap_only(struct list_head *pagelist)
>
> I'm not properly reviewing these patches at the moment but am taking a
> quick look as I play some catch up on linux-mm.
>
> I think it's worth pointing out that this potentially will confuse
> reclaim. Lets say a process is being migrated to another node and it
> gets unmapped like this then some heuristics will change.
>
> 1. If the page was referenced prior to the unmapping then it should be
> activated if the page reached the end of the LRU due to the checks
> in page_check_references(). If the process has been unmapped for
> migrate-on-fault, the pages will instead be reclaimed.
>
> 2. The heuristic that applies pressure to slab pages if pages are mapped
> is changed. Prior to migrate-on-fault sc->nr_scanned is incremented
> for mapped pages to increase the number of slab pages scanned to
> avoid swapping. During migrate-on-fault, this pressure is relieved
>
> 3. zone_reclaim_mode in default mode will reclaim pages it would
> previously have skipped over. It potentially will call shrink_zone more
> for the local node than falling back to other nodes because it thinks
> most pages are unmapped. This could lead to some trashing.
>
> It may not even be a major problem but it's worth thinking about. If it
> is a problem, it will be necessary to account for migrate-on-fault pages
> similar to mapped pages during reclaim.

I can see other serious issues with this approach:

4. Putting a lot of pages in the swap cache ends up allocating
swap space. This means this NUMA migration scheme will only
work on systems that have a substantial amount of memory
represented by swap space. This is highly unlikely on systems
with memory in the TB range. On smaller systems, it could drive
the system out of memory (to the OOM killer), by "filling up"
the overflow swap with migration pages instead.

5. In the long run, we want the ability to migrate transparent
huge pages as one unit. The reason is simple, the performance
penalty for running on the wrong NUMA node (10-20%) is on the
same order of magnitude as the performance penalty for running
with 4kB pages instead of 2MB pages (5-15%).

Breaking up large pages into small ones, and having khugepaged
reconstitute them on a random NUMA node later on, will negate
the performance benefits of both NUMA placement and THP.

In short, while this approach made sense when Lee first proposed
it several years ago (with smaller memory systems, and before Linux
had transparent huge pages), I do not believe it is an acceptable
approach to NUMA migration any more.

We really want something like PROT_NONE or PTE_NUMA page table
(and page directory) entries, so we can avoid filling up swap
space with migration pages and have the possibility of migrating
transparent huge pages in one piece at some point.

In other words, NAK to this patch

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