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Re: [Xen-devel] RE: Reducing I/O introduced domain scheduling

To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>, "Dong, Eddie" <eddie.dong@xxxxxxxxx>
Subject: Re: [Xen-devel] RE: Reducing I/O introduced domain scheduling
From: Keir Fraser <keir@xxxxxxx>
Date: Tue, 12 Oct 2010 10:18:41 +0100
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>
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Thread-topic: [Xen-devel] RE: Reducing I/O introduced domain scheduling
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No, you can't  vmexit on a fence. I don't know whether that matters, so long
as buffered writes get flushed before the guest can observe their effects
(presumably via some kind of I/O read). Agree that generalising the buffered
I/O concept feels a bit dodgy however.

 -- Keir

On 12/10/2010 10:15, "Paul Durrant" <Paul.Durrant@xxxxxxxxxx> wrote:

> Just wondering... does Xen/can Xen take VM exits on fences? If not then I
> don't see you could safely buffer MMIO writes.
>   Paul
>> -----Original Message-----
>> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-devel-
>> bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Dong, Eddie
>> Sent: 12 October 2010 02:12
>> To: Keir Fraser
>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; Dong, Eddie; Zhang, Xiantao
>> Subject: [Xen-devel] Reducing I/O introduced domain scheduling
>> Keir:
>> When running vConsolidation on top of Xen in  a 4-core
>> platform, we noticed the I/O introduced scheduling per CPU is ~3K
>> Hz, which seems to be too frequent and cause frequent involve of
>> domain 0 / Qemu, which may polute cache of the guest and thus
>> increase CPI (cycle per instruction).
>> We are thinking if we can reduce the domin switch here, and
>> think the output of I/O can be buffered and return immediately. The
>> buffered I/O can be flushed out at next IN emulation (or any
>> Hypervisor emulated I/O) or timeout such as 10 or 100 us to
>> guarantee minimal response.
>> Ideally it can cover both PIO & MMIO, but we can start from
>> PIO.
>> How do you think of that?
>> Thx, Eddie
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