[PATCH 3/3] exa/mixed: Exclude frontbuffer from deferred pixmap handling.

[Michel Dänzer]

On Mon, 2011-01-17 at 21:45 +0100, Maarten Maathuis wrote: 
> 2010/12/20 Michel Dänzer <michel at daenzer.net>:
> > On Mon, 2010-12-20 at 15:54 +0100, Maarten Maathuis wrote:
> >> 2010/12/20 Michel Dänzer <michel at daenzer.net>:
> >> > On Mon, 2010-12-20 at 15:46 +0100, Maarten Maathuis wrote:
> >> >> 2010/12/14 Michel Dänzer <michel at daenzer.net>:
> >> >> > On Mon, 2010-12-13 at 19:42 +0100, Maarten Maathuis wrote:
> >> >> >> - Apps like xterm can trigger a lot of fallback rendering.
> >> >> >> - This can lead to (annoyingly) high latencies, because you
> >> >> >>   have to wait for the block handler.
> >> >> >> - You need a driver that doesn't directly access the front
> >> >> >>   buffer to trigger this (NV50+ nouveau for example).
> >> >> >> - Repeatingly doing dmesg on an xterm with a bitmap font
> >> >> >>   will reveal that you never see part of the text.
> >> >> >> - I have recieved at least one complaint in the past of slow
> >> >> >>   terminal performance, which was related to core font
> >> >> >>   rendering.
> >> >> >> - This does sacrifice some throughput, not sure how much,
> >> >> >
> >> >> > Shouldn't be hard to measure.
> >> >>
> >> >> I did a little test (catting a saved copy of dmesg) and the throughput
> >> >> loss is about 25%.
> >> >
> >> > What are the absolute numbers?
> >>
> >> Roughly 250 ms vs 330 ms (error margin is about 20-30 ms if i had to guess).
> >
> > That seems rather inaccurate, can you try something at least an order of
> > magnitude bigger?
> 
> Forgot about it for a while, but it remains 33% slower, for 10 times
> the text. Times are typically 2.7 - 2.8 s vs 3.6 - 3.7 s.

Okay, thanks. I guess this might be acceptable for a substantial
decrease in latency, but I can't help wondering if we couldn't get that
with less if any sacrifice in throughput. Have you tried or thought
about anything else? Some random ideas offhand:

      * Limit the amount of deferred dirtiness, be it wall clock based
        or even just a simple counter of deferred ops. 
      * Flushing the deferred dirtiness in other places in addition to
        (or instead of) the BlockHandler, e.g. a flush callback.


-- 
Earthling Michel Dänzer           |                http://www.vmware.com
Libre software enthusiast         |          Debian, X and DRI developer