Re: [Qemu-devel] [PATCH] implement moderation registers for e1000

[Paolo Bonzini]

Il 08/02/2013 11:20, Luigi Rizzo ha scritto:
> On Fri, Feb 8, 2013 at 2:02 AM, Stefan Hajnoczi <address@hidden
> <mailto:address@hidden>> wrote:
> 
>     On Wed, Feb 06, 2013 at 03:23:41PM +0100, Luigi Rizzo wrote:
>     > The following patch implements interrupt moderation registers
>     > for the e1000 adapter. These feature is normally used by OS
>     > drivers, and their implementation improves performance significantly,
>     > especially on the transmit path.
>     > The feature can be disabled through a command line option.
>     > We have seen only benefits in throughput, while latency slightly
>     > increases (that is an inherent feature of interrupt moderation)
>     > because very short delays cannot be emulated precisely.
>     >
>     > For those curious on performance, there is a set of measurements
>     > (of this and other changes that we will post separately) at
>     >
>     > http://info.iet.unipi.it/~luigi/research.html#qemu
> 
>     http://info.iet.unipi.it/~luigi/papers/20130206-qemu.pdf is 404.
> 
> 
> sorry, fixed now.
> And, will resubmit a fixed patch with uninit and fixed braces in the
> if() statement.
> 
> I am happy to make this default to off. But it would be good if you could
> actually give it a try. Note that linux and FreeBSD (and presumably windows)
> do use moderation by default so enabling the emulation of the
> registers makes the guest OS behave differently (and closer to bare metal).
> 
> To test that the patch itself does not cause regression in the emulator
> one should also turn off moderation in the guest (one of the tests i
> have run).

I think making the default on is fine, but you need to add compatibility
options to leave it off in older machine types (pc-1.4 and earlier).

Paolo