Re: [RFC/T/D][PATCH 2/2] Linux/Guest cooperative unmapped page cachecontrol

From: Avi Kivity
Date: Mon Jun 14 2010 - 11:35:09 EST

On 06/14/2010 06:12 PM, Dave Hansen wrote:
On Mon, 2010-06-14 at 14:18 +0530, Balbir Singh wrote:
1. A slab page will not be freed until the entire page is free (all
slabs have been kfree'd so to speak). Normal reclaim will definitely
free this page, but a lot of it depends on how frequently we are
scanning the LRU list and when this page got added.
You don't have to be freeing entire slab pages for the reclaim to have
been useful. You could just be making space so that _future_
allocations fill in the slab holes you just created. You may not be
freeing pages, but you're reducing future system pressure.

Depends. If you've evicted something that will be referenced soon, you're increasing system pressure.

If unmapped page cache is the easiest thing to evict, then it should be
the first thing that goes when a balloon request comes in, which is the
case this patch is trying to handle. If it isn't the easiest thing to
evict, then we _shouldn't_ evict it.

Easy to evict is just one measure. There's benefit (size of data evicted), cost to refill (seeks, cpu), and likelihood that the cost to refill will be incurred (recency).

It's all very complicated. We need better information to make these decisions. For one thing, I'd like to see age information tied to objects. We may have two pages that were referenced in wildly different times be next to each other in LRU order. We have many LRUs, but no idea of the relative recency of the tails of those LRUs.

If each page or object had an age, we could scale those ages by the benefit from reclaim and cost to refill and make a better decision as to what to evict first. But of course page->age means increasing sizeof struct page, and we can only approximate its value by scanning the accessed bit, not determine it accurately (unlike the other objects managed by the cache).

error compiling committee.c: too many arguments to function

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