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Re: [Xen-devel] [RFC PATCH 0/2] ASID: Flush by ASID

To: Wei Wang2 <wei.wang2@xxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [RFC PATCH 0/2] ASID: Flush by ASID
From: Keir Fraser <keir@xxxxxxx>
Date: Wed, 12 Jan 2011 13:38:04 +0000
Cc: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
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Thread-topic: [Xen-devel] [RFC PATCH 0/2] ASID: Flush by ASID
User-agent: Microsoft-Entourage/
Our gut feeling has always been that the major benefit is having two ASIDS,
allowing one for host and one for current guest and thus avoiding TLB flush
on every VM entry/exit. Unless your TLB is very large, or guest vcpus run
only for very short periods, it's likely that a heavy guest workload
displaces all other ASIDs (guest VCPUs) from the TLB anyway.

We're interested in benchmark numbers that can disprove the gut feeling, of

 -- Keir

On 12/01/2011 13:23, "Wei Wang2" <wei.wang2@xxxxxxx> wrote:

> Keir,
> Sure, that is a good question :) .
> Actually finding a benchmark that scales with asid well is not quite easy.
> Benckmark like Kernbench which has large working set will occupy all tls
> entries by its own asid. In this case, even disabling asid is not harmful.
> We only tested single guest with multiple vcpus. Maybe using multiple guests
> or other benchmarks will show a better result?
> Thanks,
> Wei
> On Wednesday 12 January 2011 13:48:49 Keir Fraser wrote:
>> It begs the question whether it's worth complicating code for an
>> optimisation with no measurable benefit, doesn't it?
>>  -- Keir
>> On 12/01/2011 12:41, "Wei Wang2" <wei.wang2@xxxxxxx> wrote:
>>> Hi Tim,
>>> Flush by ASID provides more flexible control of tlb flushing. The most
>>> advantage is to allow hypervisor to flush tagged tlb selectively. Using
>>> this feature, HV is able to flush tlb entries associated with a guest VM
>>> directly instead of allocating a new asid . The whole tlb flush will also
>>> be reduced by reducing asid allocation.
>>> So far, we did not measure drastic performance improvement in testing
>>> with kernbench and X11perf. Actually, we found out that, reducing tlb
>>> flushes accompanying with vmrun does not improve performance very much.
>>> we sent out a patch to optimize hvm_flush_guest_tlbs last week, which
>>> reduces over 90% tlb flushes for vmrun, and we even cannot see
>>> signification speedup with it. Maybe, the latency of vmrun is too big so
>>> that the overhead of tlb flush is negligible?
>>> Thanks,
>>> Wei
>>> On Wednesday 12 January 2011 11:17:00 Tim Deegan wrote:
>>>> At 17:55 +0000 on 11 Jan (1294768552), Wei Wang2 wrote:
>>>>> Future AMD SVM supports a new feature called flush by ASID. The idea is
>>>>> to allow CPU to flush TLBs associated with the ASID assigned to guest
>>>>> VM. So hypervisor doesn't have to reassign a new ASID in order to flush
>>>>> guest's VCPU.  Please review it.
>>>> What advantage does the new system have?  Intuitively it seems like it
>>>> might be a tiny bit fairer and a tiny bit faster (by explicitly flushing
>>>> instead of relying on LRO) but I'm not convinced that it will be visible
>>>> in macro-benchmarks.  Have you measured it?
>>>> Cheers,
>>>> Tim.
>>>>> Thanks,
>>>>> Wei
>>>>> Signed-off-by: Wei Huang <wei.huang2@xxxxxxx>
>>>>> Signed-off-by: Wei Wang <wei.wang2@xxxxxxx>
>>>>> --
>>>>> Advanced Micro Devices GmbH
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>>>>> Alberto Bozzo, Andrew Bowd
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