Re: [PATCH] fix free swap cache latency

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Andrew Morton wrote:
Hugh Dickins <[email protected]> wrote:

			(*zap_work)--;
			continue;
		}
+
+		(*zap_work) -= PAGE_SIZE;


Sometimes we subtract 1 from zap_work, sometimes PAGE_SIZE.  It's in units
of bytes, so PAGE_SIZE is correct.  Although it would make sense to
redefine it to be in units of PAGE_SIZE.  What's up with that?


Subtracting 1 if there is no work to do for that cacheline entry.

Even better, define it in units of "approximate number of touched
cachelines".  After all, it is a sort-of-time-based thing.


Yeah that was the rough intention, but I never actually measured
to see whether the results were right.

The pte_none case touches about 1/32 of a cacheline on my P4
(add a bit for setup costs, subtract a bit for linear prefetchable
access over multiple lines).

pte_present touches the pte, the page once or twice -- first
directly, then from mmu gather, the vma and the mapping (mostly be
the same though so cost is small), a whole host of locks and atomic
operations (put_page_testzero, lru_lock, tree_lock, page_remove_rmap,
zone->lock), an IPI to other CPUs, tlb invalidates etc. some things
batched, some not.

So it gets hard to count, especially if you have other CPUs contending
the same locks. I suspect the per-page cost is not really 128 cache
misses most of the time, but it was just a number I pulled out. Anyone
can feel free to turn the knob if they feel they have a better idea.

--
SUSE Labs, Novell Inc.
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