linux-kernel - Re: [PATCH] Avoid useless inodes and dentries reclamation

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Date:	Thu, 29 Aug 2013 11:36:10 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
CC:	Dave Chinner <david@...morbit.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.cz>, Dave Chinner <dchinner@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Avoid useless inodes and dentries reclamation

The new shrinker infrastructure in mmotm looks like it will make this
problem worse.

old code:
shrink_slab()
	for_each_shrinker {
		do_shrinker_shrink(); // one per batch
			prune_super()
				grab_super_passive()
	}
}

Which means we've got at _most_ one grab_super_passive() per batch.  The
new code is something like this:

shrink_slab()
{
	list_for_each_entry(shrinker, &shrinker_list, list) {
                for_each_node_mask(... shrinkctl->nodes_to_scan) {
			shrink_slab_node()
		}
	}
}

shrink_slab_node()
{
        max_pass = shrinker->count_objects(shrinker, shrinkctl);
	// ^^ does grab_super_passive()
	...
	while (total_scan >= batch_size) {
		ret = shrinker->scan_objects(shrinker, shrinkctl);
		// ^^ does grab_super_passive()
	}
}

We've got an extra grab_super_passive()s in the case where we are
actually doing a scan, plus we've got the extra for_each_node_mask()
loop.  That means even more lock acquisitions in the multi-node NUMA
case, which is exactly where we want to get rid of global lock acquisitions.

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