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xen-devel
[Xen-devel] Re: [patch 0/6] Guest page hinting version 7.
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To: |
Martin Schwidefsky <schwidefsky@xxxxxxxxxx> |
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Subject: |
[Xen-devel] Re: [patch 0/6] Guest page hinting version 7. |
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From: |
Jeremy Fitzhardinge <jeremy@xxxxxxxx> |
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Date: |
Fri, 03 Apr 2009 11:19:24 -0700 |
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Cc: |
akpm@xxxxxxxx, Nick Piggin <nickpiggin@xxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, linux-mm@xxxxxxxxx, frankeh@xxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, virtualization@xxxxxxxxxxxxxx, hugh@xxxxxxxxxxx, virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx |
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Fri, 03 Apr 2009 11:19:57 -0700 |
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Martin Schwidefsky wrote:
This is the basic idea of guest page hinting. Let the host memory
manager make it decision based on the data it has. That includes page
age determined with a global LRU list, page age determined with a
per-guest LRU list, i/o rates of the guests, all kind of policy which
guest should have how much memory.
Do you look at fault rates? Refault rates?
The page hinting comes into play
AFTER the decision has been made which page to evict. Only then the host
should look at the volatile vs. stable page state and decide what has
to be done with the page. If it is volatile the host can throw the page
away because the guest can recreate it with LESS effort. That is the
optimization.
Yes, and its good from that perspective. Do you really implement it
purely that way, or do you bias the LRU to push volatile and free pages
down the end of the LRU list in preference to pages which must be
preserved? If you have a small bias then you can prefer to evict easily
evictable pages compared to their near-equivalents which require IO.
But with page hinting you don't have to even ask. Just take the pages
if you need them. The guest already told you that you can have them by
setting the unused state.
Yes. But it still depends on the guest. A very helpful guest could
deliberately preswap pages so that it can mark them as volatile, whereas
a less helpful one may keep them persistent and defer preswapping them
until there's a good reason to do so. Host swapping and page hinting
won't put any apparent memory pressure on the guest, so it has no reason
to start preswapping even if the overall system is under pressure.
Ballooning will expose each guest to its share of the overall system
memory pressure, so they can respond appropriately (one hopes).
J
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