* Re: turn off writable page tables
[not found] ` <20060803204205.GA29014@mdday.raleigh.ibm.com>
@ 2006-08-09 21:15 ` Andrew Theurer
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From: Andrew Theurer @ 2006-08-09 21:15 UTC (permalink / raw)
To: Mike D. Day
Cc: Zachary Amsden, Ian Pratt, xen-devel, Andi Kleen, virtualization
Mike D. Day wrote:
> On 02/08/06 10:21 +0100, Keir Fraser wrote:
>
>> I wonder how it will interact with our late-pin/early-unpin model
>> where we can directly write to pagetables before they are first used
>> and also while they are being finally destroyed. It may be that if we
>> use explicit batching and turn off that pinning logic we will not
>> affect performance much, but we'll need to do some performance analysis.
>
> Where are we on Andrew's Patch now? I think Andrew's Patch is a good
> solution for the time being until we can get batching, which may be
> too disruptive for 3.03.
> Mike
I would agree :) Honestly, there's does not appear to be any evidence
that batching of PTE updates, on typical applications, appear to have
any performance advantage, but the current batching implementation does
hurt scalability, and what really worries me, keeping it means keeping
around a lot of code in xen mm that has to catch coherency issues (all
the cleanup_writable_pagetable() calls and similar checks). I rather
have writable PTs removed completely now (and assume they don't come
back), then add explicit batching/multi-calls to specific parts of the
guests' kernels (which should not require extra code in Xen mm like the
coherency checks for writable PTs) -and only if they show to benefit
from it.
-Andrew
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