Re: [Qemu-devel] [PATCH 06/22] ehci: Speed up the timer of raising int from the async schedule

[Hans de Goede]

Hi,

On 10/15/2012 01:17 PM, Gerd Hoffmann wrote:
> On 10/15/12 12:38, Hans de Goede wrote:
> > Often the guest will queue up new packets in response to a packet, in the
> > async schedule with its IOC flag set, completing. By speeding up the
> > frame-timer, we notice these new packets earlier. This increases the
> > speed (MB/s) of a Linux guest reading from a USB mass storage device by a
> > factor of 1.15 on top of the "Improve latency of interrupt delivery"
> > speed-ups, both with and without input pipelining enabled.
>
> Why not just set async_stepdown to 0?

We already do that whenever we run a package completion (it get sets when
we move to the executing stage). What this patch does is request the
frame timer to run again in 500 usecs instead of after 1 ms, thus making
us see and process async transfers faster when they are queued up in
response to just completed packages (which we've told the guest about with
the int interrupt). This makes the USB-bus / device idle time between
any 2 transfers of the 3 transfer involving USB storage BOT time shorter,
thereby speeding things up.

Regards,

Hans