Messages in this thread | | | Date | Sun, 27 Nov 2011 15:50:22 -0500 | From | Rik van Riel <> | Subject | Re: [PATCH 4/5] mm: compaction: Determine if dirty pages can be migreated without blocking within ->migratepage |
| |
On 11/24/2011 07:21 AM, Mel Gorman wrote: > On Thu, Nov 24, 2011 at 02:19:43AM +0100, Andrea Arcangeli wrote:
>> But funny thing grow_dev_page already sets __GFP_MOVABLE. That's >> pretty weird and it's probably source of a few not movable pages in >> the movable block. But then many bh are movable... most of them are, >> it's just the superblock that isn't. >> >> But considering grow_dev_page sets __GFP_MOVABLE, any worry about pins >> from the fs on the block_dev.c pagecache shouldn't be a concern... >> > > Except in quantity. We can cope with some pollution of MIGRATE_MOVABLE > but if it gets excessive, it will cause a lot of trouble. Superblock > bh's may not be movable but there are not many of them and they are > long lived.
We're potentially doomed either way :)
If we allocate a lot of movable pages in non-movable blocks, we can end up with a lot of slightly polluted blocks even after reclaiming all the reclaimable page cache.
If we allocate a few non-movable pages in movable blocks, we can end up with the same situation.
Either way, we can potentially end up with a lot of memory that cannot be defragmented.
Of course, it could take the mounting of a lot of filesystems for this problem to be triggered, but we know there are people doing that.
>> __GFP_MOVABLE missing block_dev also was not >> so common and it most certainly contributed to a reclaim more >> aggressive than it would have happened with that fix. I think you can >> push things one at time without urgency here, and I'd prefer maybe if >> block_dev patch is applied and the other reversed in vmscan.c or >> improved to start limiting only if we're above 8*high or some >> percentage check to allow a little more reclaim than rc2 allows > > The limiting is my current preferred option - at least until it is > confirmed that it really is ok to mark block_dev pages movable and that > Rik is ok with the revert.
I am fine with replacing the compaction checks with free limit checks. Funny enough, the first iteration of the patch I submitted to limit reclaim used a free limit check :)
I also suspect we will want to call shrink_slab regardless of whether or not a memory zone is already over its free limit for direct reclaim, since that has the potential to free an otherwise unmovable page.
>> (i.e. no reclaim at all which likely results in a failure in hugepage >> allocation). Not unlimited as 3.1 is ok with me but if kswapd can free >> a percentage I don't see why reclaim can't (consdiering more free >> pages in movable pageblocks are needed to succeed compaction). The >> ideal is to improve the compaction rate and at the same time reduce >> reclaim aggressiveness. Let's start with the parts that are more >> obviously right fixes and that don't risk regressions, we don't want >> compaction regressions :). >> > > I don't think there are any "obviously right fixes" right now until the > block_dev patch is proven to be ok and that reverting does not regress > Rik's workload. Going to take time.
Ironically the test Andrea is measuring THP allocations with (dd from /dev/sda to /dev/null) is functionally equivalent to me running KVM guests with cache=writethrough directly from a block device.
The difference is that Andrea is measuring THP allocation success rate, while I am watching how well the programs (and KVM guests) actually run.
Not surprisingly, swapping out the working set has a pretty catastrophic effect on performance, even if it helps THP allocation success :)
-- All rights reversed
| |