On 28/07/2018 17:46, Chris Wilson wrote:
Looking at the distribution of i915_wait_request for a set of GL
What was the set?
benchmarks, we see:
broadwell# python bcc/tools/funclatency.py -u i915_wait_request
usecs : count distribution
0 -> 1 : 29184 |****************************************|
2 -> 3 : 5767 |******* |
4 -> 7 : 3000 |**** |
8 -> 15 : 491 | |
16 -> 31 : 140 | |
32 -> 63 : 203 | |
64 -> 127 : 543 | |
128 -> 255 : 881 |* |
256 -> 511 : 1209 |* |
512 -> 1023 : 1739 |** |
1024 -> 2047 : 22855 |******************************* |
2048 -> 4095 : 1725 |** |
4096 -> 8191 : 5813 |******* |
8192 -> 16383 : 5348 |******* |
16384 -> 32767 : 1000 |* |
32768 -> 65535 : 4400 |****** |
65536 -> 131071 : 296 | |
131072 -> 262143 : 225 | |
262144 -> 524287 : 4 | |
524288 -> 1048575 : 1 | |
1048576 -> 2097151 : 1 | |
2097152 -> 4194303 : 1 | |
broxton# python bcc/tools/funclatency.py -u i915_wait_request
usecs : count distribution
0 -> 1 : 5523 |************************************* |
2 -> 3 : 1340 |********* |
4 -> 7 : 2100 |************** |
8 -> 15 : 755 |***** |
16 -> 31 : 211 |* |
32 -> 63 : 53 | |
64 -> 127 : 71 | |
128 -> 255 : 113 | |
256 -> 511 : 262 |* |
512 -> 1023 : 358 |** |
1024 -> 2047 : 1105 |******* |
2048 -> 4095 : 848 |***** |
4096 -> 8191 : 1295 |******** |
8192 -> 16383 : 5894 |****************************************|
16384 -> 32767 : 4270 |**************************** |
32768 -> 65535 : 5622 |************************************** |
65536 -> 131071 : 306 |** |
131072 -> 262143 : 50 | |
262144 -> 524287 : 76 | |
524288 -> 1048575 : 34 | |
1048576 -> 2097151 : 0 | |
2097152 -> 4194303 : 1 | |
Picking 20us for the context-switch busyspin has the dual advantage of
catching most frequent short waits while avoiding the cost of a context
switch. 20us is a typical latency of 2 context-switches, i.e. the cost
of taking the sleep, without the secondary effects of cache flushing.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Sagar Kamble <sagar.a.kamble@xxxxxxxxx>
Cc: Eero Tamminen <eero.t.tamminen@xxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Cc: Ben Widawsky <ben@xxxxxxxxxxxx>
Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
Cc: Michał Winiarski <michal.winiarski@xxxxxxxxx>
---
drivers/gpu/drm/i915/Kconfig.profile | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/i915/Kconfig.profile b/drivers/gpu/drm/i915/Kconfig.profile
index 63cb744d920d..de394dea4a14 100644
--- a/drivers/gpu/drm/i915/Kconfig.profile
+++ b/drivers/gpu/drm/i915/Kconfig.profile
@@ -14,7 +14,7 @@ config DRM_I915_SPIN_REQUEST_IRQ
config DRM_I915_SPIN_REQUEST_CS
int
- default 2 # microseconds
+ default 20 # microseconds
help
After sleeping for a request (GPU operation) to complete, we will
be woken up on the completion of every request prior to the one
I'd be more tempted to pick 10us given the histograms. It would avoid
wasting cycles on Broadwell and keep the majority of the benefit on Broxton.
However.. it also raises the question if we perhaps want to have this
initialized per-platform at runtime.. ? That would open up the way of
auto-tuning it, if the goal is to eliminate the low part of the histogram.
Also, please add to the commit what kind of perf/watt or something
effect on the benchmarks we get with it.
Regards,
Tvrtko
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