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Re: [Qemu-devel] interrupt mitigation for e1000

From: Paolo Bonzini
Subject: Re: [Qemu-devel] interrupt mitigation for e1000
Date: Wed, 25 Jul 2012 12:12:55 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120615 Thunderbird/13.0.1

Il 25/07/2012 11:56, Luigi Rizzo ha scritto:
> On Wed, Jul 25, 2012 at 11:53:29AM +0300, Avi Kivity wrote:
>> On 07/24/2012 07:58 PM, Luigi Rizzo wrote:
>>> I noticed that the various NIC modules in qemu/kvm do not implement
>>> interrupt mitigation, which is very beneficial as it dramatically
>>> reduces exits from the hypervisor.
>>> As a proof of concept i tried to implement it for the e1000 driver
>>> (patch below), and it brings tx performance from 9 to 56Kpps on
>>> qemu-softmmu, and from ~20 to 140Kpps on qemu-kvm.
>>> I am going to measure the rx interrupt mitigation in the next couple
>>> of days.
>>> Is there any interest in having this code in ?
>> Indeed.  But please drop the #ifdef MITIGATIONs.
> Thanks for the comments. The #ifdef block MITIGATION was only temporary to
> point out the differences and run the performance comparisons.
> Similarly, the magic thresholds below will be replaced with
> appropriately commented #defines.
> Note:
> On the real hardware interrupt mitigation is controlled by a total of four
> registers (TIDV, TADV, RIDV, RADV) which control it with a granularity
> of 1024ns , see
> http://www.intel.com/content/dam/doc/manual/pci-pci-x-family-gbe-controllers-software-dev-manual.pdf
> An exact emulation of the feature is hard, because the timer resolution we
> have is much coarser (in the ms range). So i am inclined to use a different
> approach, similar to the one i have implemented, namely:
> - the first few packets (whether 1 or 4 or 5 will be decided on the host)
>   report an interrupt immediately;
> - subsequent interrupts are delayed through qemu_bh_schedule_idle()

qemu_bh_schedule_idle() is really a 10ms timer.


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