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Re: [Xen-devel] Re: Even faster page copy for Xen?

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] Re: Even faster page copy for Xen?
From: Dulloor <dulloor@xxxxxxxxx>
Date: Mon, 9 Aug 2010 10:47:37 -0700
Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
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On Fri, Aug 6, 2010 at 12:57 AM, Jan Beulich <JBeulich@xxxxxxxxxx> wrote:
>>>> On 15.07.10 at 20:15, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> wrote:
>> Hi Jan, Keir --
>> My x86 assembly skills are much too poor to carefully evaluate
>> and, if of value, implement this in Xen but given your previous
>> interest, such as:
>> http://xenbits.xensource.com/xen-unstable.hg?rev/8de4b4e9a435
>> the following might be worth looking at.
>> Intel has just posted memcpy improvements for glibc for recent
>> popular Intel processor families here:
>> http://article.gmane.org/gmane.comp.lib.glibc.alpha/15278
>> The preface to the above patch looks very enticing...
> I'm not sure how much of this applies to the much more specific
> case of copying pages... Additionally, I don't think trying to
> use XMM registers in Xen would be a good idea.
Why would you say using xmm/sse in Xen is a bad idea ? We already have a
copy_page_sse2 (in copy_page.S) in our code base and available (by default)
for x86_64. Is it a bad idea to use that ?

>> Semi-related, I wonder if you know, if there were a
>> "copy_page_from_other_node()" to be used if the
>> caller is fairly sure that the page is being copied
>> between nodes, could this be made significantly faster
>> than a normal copy_page()?
> I would think that this should mostly be taken care of by
> using non-temporal stores (non-temporal loads unfortunately
> aren't available without using XMM registers). The only other
> meaningful tuning one could do would be to increase the
> prefetch distances and grow the distance between loads and
> stores. The latter would require the use of more registers
> and hence have other drawbacks.
> Jan
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