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

[Maarten Maathuis]

2011/1/18 Michel Dänzer <michel at daenzer.net>:
> 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.
>

I kind of went for the "best" solution in terms of latency
(considering it doesn't happen all that often for most people), the
second best would probably be a frontbuffer area counter, requiring a
certain square pixel damaged area before flushing. The downside is
that latency while typing in a console will still be there. Some kind
of maximum latency timer might work as well, although i don't know if
the xserver has those. But that won't reduce latency to a minimum in
low throughput situations.

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



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