[PATCH] glretrace: Use GL_TIMESTAMP (if available) for CPU profiling.

[Carl Worth]

José Fonseca <jose.r.fonseca at gmail.com> writes:
> Won't the CPU and GPU timings match 1:1 the same then?

Not really. The only place they will match (as the patch is currently
written) is on the start time of drawing calls. And that's a part of the
patch where we may want to do something different.

> The CPU start/end times are useful information. If they are always out
> of sync with the GPU then instead of discarding them ... 

I'm not discarding any times. I'm still recording CPU start times at the
same point as before. The only difference is that I'm using a reliable
clock to record those times.

>                                                  ... we should update
> the GUI so that CPU and GPU time axis can vary independent.

I'm not sure what you mean here, nor how it could be useful. There
doesn't exist some magic third clock by which you can correlate these
two unrelated clocks.

> Also, why are GPU counters doomed to be always be unreliable?

Just the opposite, in fact. It's the GPU counter that is reliable. With
current Intel hardware, the GPU counter gives us a nice, monotonic,
fixed-frequency clock. It's the CPU clock that changes frequency, etc.

My question is why would we ever use independent clocks subject to
drift? The current code attempts to re-synchronize the clocks once per
frame. So there's one call per frame where things might actually be
consistent. But all bets are off after that. And in practice, the drift
I saw was obvious—GPU calls were showing up as starting before the
corresponding CPU call had even started. That drift was large enough to
make me doubt any other CPU-GPU timing difference anywhere in the GUI.

-Carl

-- 
carl.d.worth at intel.com
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 197 bytes
Desc: not available
URL: <http://lists.freedesktop.org/archives/apitrace/attachments/20121115/ea19b18d/attachment.pgp>