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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 10:44:48 +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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Are there any such scenarios that could reveal not-allowed-by-hardware-spec
buffering? If so then that would make the optimisation invalid. But anyway,
it seems that the Intel guys should just fix their test setup to get their
system disk using PV mode.

 -- Keir

On 12/10/2010 10:30, "Paul Durrant" <Paul.Durrant@xxxxxxxxxx> wrote:

> My concern is a read from a non-MMIO page following a write to an MMIO page.
>   Paul
>> -----Original Message-----
>> From: Keir Fraser [mailto:keir.xen@xxxxxxxxx] On Behalf Of Keir
>> Fraser
>> Sent: 12 October 2010 10:19
>> To: Paul Durrant; Dong, Eddie
>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; Zhang, Xiantao
>> Subject: Re: [Xen-devel] RE: Reducing I/O introduced domain
>> scheduling
>> 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
>>>> _______________________________________________
>>>> Xen-devel mailing list
>>>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>>>> http://lists.xensource.com/xen-devel
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