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

From: Avi Kivity
Date: Mon Jun 14 2010 - 08:40:39 EST

On 06/14/2010 11:48 AM, Balbir Singh wrote:

In this case the order is as follows

1. First we pick free pages if any
2. If we don't have free pages, we go after unmapped page cache and
slab cache
3. If that fails as well, we go after regularly memory

In the scenario that you describe, we'll not be able to easily free up
the frequently referenced page from /etc/*. The code will move on to
step 3 and do its regular reclaim.
Still it seems to me you are subverting the normal order of reclaim.
I don't see why an unmapped page cache or slab cache item should be
evicted before a mapped page. Certainly the cost of rebuilding a
dentry compared to the gain from evicting it, is much higher than
that of reestablishing a mapped page.

Subverting to aviod memory duplication, the word subverting is

Right, should have used a different one.

let me try to reason a bit. First let me explain the

Memory is a precious resource in a consolidated environment.
We don't want to waste memory via page cache duplication
(cache=writethrough and cache=writeback mode).

Now here is what we are trying to do

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.
2. In the case of page cache (specifically unmapped page cache), there
is duplication already, so why not go after unmapped page caches when
the system is under memory pressure?

In the case of 1, we don't force a dentry to be freed, but rather a
freed page in the slab cache to be reclaimed ahead of forcing reclaim
of mapped pages.

Sounds like this should be done unconditionally, then. An empty slab page is worth less than an unmapped pagecache page at all times, no?

Does the problem statement make sense? If so, do you agree with 1 and
2? Is there major concern about subverting regular reclaim? Does
subverting it make sense in the duplicated scenario?

In the case of 2, how do you know there is duplication? You know the guest caches the page, but you have no information about the host. Since the page is cached in the guest, the host doesn't see it referenced, and is likely to drop it.

If there is no duplication, then you may have dropped a recently-used page and will likely cause a major fault soon.

error compiling committee.c: too many arguments to function

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