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Re: [Xen-devel] [PATCH] turn off writable page tables

To: Andrew Theurer <habanero@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] turn off writable page tables
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Thu, 27 Jul 2006 16:30:43 +0100
Cc: Ian Pratt <m+Ian.Pratt@xxxxxxxxxxxx>, Gerd Hoffmann <kraxel@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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On 27 Jul 2006, at 15:43, Andrew Theurer wrote:

So, the threshold appears to be around 4 PTEs/page. I was a little shocked at first how low this number is, but considering the near identical performance with the various workloads, this make sense. All of the workloads had the vast majority of writable pages flushed with just 2 PTEs/page changed and a handful with more PTEs/page changed. It would not surprise me if the overall average was around 4 PTEs/page.

I am having a hard time finding any "enterprise" workloads which have a lot of PTEs/page right before fork. If anyone can point me to some, that would be great.

I will look into batching next, but I am curious if simply using a hypercall in stead of write fault + emulate will make any difference at all. I'll try that first, then implement the batched update. Eventually a hypercall which does more would be nice, but I guess we'll have to convince the Linux maintainers it's a good idea.

The obvious thing to do is emulate the first 4 updates to a particular page, and only then switch to batched mode. Slows down the batched path a bit, but stops it firing in many cases where it is no help.

 -- Keir

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