Re: [Qemu-devel] [PATCH 5/9] mm: compaction: Acquire the zone->lru_lock as late as possible

[Rafael Aquini]

On Fri, Sep 21, 2012 at 11:46:19AM +0100, Mel Gorman wrote:
> Compactions migrate scanner acquires the zone->lru_lock when scanning a range
> of pages looking for LRU pages to acquire. It does this even if there are
> no LRU pages in the range. If multiple processes are compacting then this
> can cause severe locking contention. To make matters worse commit b2eef8c0
> (mm: compaction: minimise the time IRQs are disabled while isolating pages
> for migration) releases the lru_lock every SWAP_CLUSTER_MAX pages that are
> scanned.
> 
> This patch makes two changes to how the migrate scanner acquires the LRU
> lock. First, it only releases the LRU lock every SWAP_CLUSTER_MAX pages if
> the lock is contended. This reduces the number of times it unnecessarily
> disables and re-enables IRQs. The second is that it defers acquiring the
> LRU lock for as long as possible. If there are no LRU pages or the only
> LRU pages are transhuge then the LRU lock will not be acquired at all
> which reduces contention on zone->lru_lock.
> 
> Signed-off-by: Mel Gorman <address@hidden>
> Acked-by: Rik van Riel <address@hidden>
> ---

Acked-by: Rafael Aquini <address@hidden>