Re: Kswapd in 3.2.0-rc5 is a CPU hog

From: Dave Chinner
Date: Wed Dec 28 2011 - 16:34:03 EST

On Tue, Dec 27, 2011 at 01:44:05PM +0900, KAMEZAWA Hiroyuki wrote:
> To me, it seems kswapd does usual work...reclaim small memory until free
> gets enough. And it seems 'dd' allocates its memory from ZONE_DMA32 because
> of gfp_t fallbacks.
> Memo.
> 1. why shrink_slab() should be called per zone, which is not zone aware.
> Isn't it enough to call it per priority ?

It is intended that it should be zone aware, but the current
shrinkers only have global LRUs and hence cannot discriminate
between objects from different zones easily. And if only a single
node/zone is being scanned, then we still have to call shirnk_slab()
to try to free objects in that zone/node, despite it's current
global scope.

I have some prototype patches that make the major slab caches and
shrinkers zone/node aware - that is the eventual goal here - but
first all the major slab cache LRUs need to be converted to be node
aware first. Then we can pass a nodemask into shrink_slab() and down
to the shrinkers so that those that have per-node LRUs can scan only
the appropriate nodes for objects to free. This is someting that I'm
working on in my spare time, but I have very little of that at the
moment, unfortunately.

> 2. what spinlock contention that perf showed ?
> And if shrink_slab() doesn't consume cpu as trace shows, why perf
> says shrink_slab() is heavy..

There isn't any spin lock contention - it's just showing how
expensive locking superblocks is when it's being done every few
microseconds for no good reason.

> 3. because 8/9 of memory is in DMA32, calling shrink_slab() frequently
> at scanning NORMAL seems to be time wasting.

Especially as the shrink_slab() calls are returning zero pages freed
every single time (i.e. the slab caches are empty). kswapd needs to
back off here, I think, or free more memory at a time. Only freeing
100 pages at a time is pretty inefficient, esp. as we have 4 orders
of magnitude more pages on the LRU and that is consuming >90% of


Dave Chinner
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