[Mesa-dev] [PATCH v2 1/1] clover: Wait for requested operation if blocking flag is set
Francisco Jerez
currojerez at riseup.net
Sat Aug 5 19:26:32 UTC 2017
Jan Vesely <jan.vesely at rutgers.edu> writes:
> Hi,
>
> thanks for detailed explanation. I indeed missed the writeBuffer part
> in specs.
>
> On Wed, 2017-08-02 at 15:05 -0700, Francisco Jerez wrote:
>> These changes are somewhat redundant and potentially
>> performance-impacting, the reason is that in the OpenCL API,
>> clEnqueueWrite* commands are specified to block until the memory
>> provided by the application as origin can be reused safely (i.e. until
>> soft_copy_op()() runs), not necessarily until the transfer to graphics
>> memory has completed (which is what hard_event::wait() will wait for).
>> OTOH reads and maps as implemented by soft_copy_op and friends are
>> essentially blocking so the wait() call is redundant in most cases.
>
> That explains a noticeable slowdown running piglit with these changes.
> I'm not sure about the read part though. I expected it to be basically
> a noop, but it changes behaviour.
I think this change would have slowed you down the most whenever the
mapping operation performed by soft_copy_op() is able to proceed
immediately, either because the buffer is idle (so the driver doesn't
stall on transfer_map()) *or* because the driver is trying to be smart
and creates a bounce buffer where data can be copied into immediately
without stalling, then inserts a pipelined GPU copy from the bounce
buffer into the real buffer. With this patch you will stall on the GPU
copy regardless (and whatever other work was already on the command
stream which might be substantial), even though it wouldn't have been
necessary in any of these cases.
> Adding clGetEventInfo(CL_EVENT_COMMAND_EXECUTION_STATUS) after a
> blocking read in one of the piglit tests surprisingly returns
> CL_QUEUED.
>
Hmm, yeah, that seems kind of debatable behaviour, although it's
definitely legit for writes, not quite sure for reads... I believe the
reason why that happens is because the CPU copy proceeds very quickly
(due to the reasons mentioned in the last paragraph), but the hard_event
is still associated with a pipe_fence synchronous with the GPU's command
stream, so it won't get signalled until the GPU catches up.
> The specs don't mention use of events with blocking read, but it does
> say that "When the read command has completed, the contents of the
> buffer that ptr points to can be used by the application." in the non-
> blocking section. I'd say that the expectation is for the event to be
> CL_COMPLETE after blocking read (at least beignet/pocl/intel-cpu-sdk
> follow this).
>
>>
>> The only reason why it might be useful to behave differently on blocking
>> transfers is that the application may have specified a user event in the
>> event dependency list, which may cause the soft_copy_op() call to be
>> delayed until the application signals the user event. In order to fix
>> that it should really only be necessary to wait for the event action to
>> be executed, not necessarily its associated GPU work.
>
> I think that another use is that non-blocking writes do not create an
> extra copy of the buffer. Thus
> clEnqueueWriteBuffer(...,cl_false, ev, ...)
> clWaitForEvents(ev)
> is more memory efficient.
>
>>
>> Last time this issue came up (yeah it's not the first time) I proposed
>> the patches below to add a mechanism to wait for the event action only,
>> feel free to include it as PATCH 0.1 and 0.2 of this series (it's been a
>> while so they may no longer apply cleanly).
>
> I think we can just add comments explaining why the blocking argument
> is ignored, until someone chooses to fix this problem
I think the problem is definitely worth fixing, and it shouldn't really
take more effort than adding comments explaining the current behaviour
;), basically just add a bunch of 'if (blocking)
hev().wait_signalled();' where the spec requires it, roughly as you had
been doing in this patch, but wait_signalled() should only stall on the
CPU work associated with the event, which should give you the same
performance as the current approach.
> and/or to
> implement proper non-blocking variants (would std::async work for
> trivial cases like ReadBuffer?)
>
> thanks,
> Jan
>
>>
>> Thank you.
>>
>> Jan Vesely <jan.vesely at rutgers.edu> writes:
>>
>> > v2: wait in map_buffer and map_image as well
>> >
>> > Signed-off-by: Jan Vesely <jan.vesely at rutgers.edu>
>> > ---
>> > Hi Aaron,
>> >
>> > yes, I think you're right, we should wait in Map* as well.
>> > If nothing else it's consistent, even if passing the flag to add_map might make it unnecessary (haven't actually checked).
>> >
>> > thanks,
>> > Jan
>> >
>> > src/gallium/state_trackers/clover/api/transfer.cpp | 30 ++++++++++++++++++++--
>> > 1 file changed, 28 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/src/gallium/state_trackers/clover/api/transfer.cpp b/src/gallium/state_trackers/clover/api/transfer.cpp
>> > index f7046253be..729a34590e 100644
>> > --- a/src/gallium/state_trackers/clover/api/transfer.cpp
>> > +++ b/src/gallium/state_trackers/clover/api/transfer.cpp
>> > @@ -295,6 +295,9 @@ clEnqueueReadBuffer(cl_command_queue d_q, cl_mem d_mem, cl_bool blocking,
>> > &mem, obj_origin, obj_pitch,
>> > region));
>> >
>> > + if (blocking)
>> > + hev().wait();
>> > +
>> > ret_object(rd_ev, hev);
>> > return CL_SUCCESS;
>> >
>> > @@ -325,6 +328,9 @@ clEnqueueWriteBuffer(cl_command_queue d_q, cl_mem d_mem, cl_bool blocking,
>> > ptr, {}, obj_pitch,
>> > region));
>> >
>> > + if (blocking)
>> > + hev().wait();
>> > +
>> > ret_object(rd_ev, hev);
>> > return CL_SUCCESS;
>> >
>> > @@ -362,6 +368,9 @@ clEnqueueReadBufferRect(cl_command_queue d_q, cl_mem d_mem, cl_bool blocking,
>> > &mem, obj_origin, obj_pitch,
>> > region));
>> >
>> > + if (blocking)
>> > + hev().wait();
>> > +
>> > ret_object(rd_ev, hev);
>> > return CL_SUCCESS;
>> >
>> > @@ -399,6 +408,9 @@ clEnqueueWriteBufferRect(cl_command_queue d_q, cl_mem d_mem, cl_bool blocking,
>> > ptr, host_origin, host_pitch,
>> > region));
>> >
>> > + if (blocking)
>> > + hev().wait();
>> > +
>> > ret_object(rd_ev, hev);
>> > return CL_SUCCESS;
>> >
>> > @@ -504,6 +516,9 @@ clEnqueueReadImage(cl_command_queue d_q, cl_mem d_mem, cl_bool blocking,
>> > &img, src_origin, src_pitch,
>> > region));
>> >
>> > + if (blocking)
>> > + hev().wait();
>> > +
>> > ret_object(rd_ev, hev);
>> > return CL_SUCCESS;
>> >
>> > @@ -538,6 +553,9 @@ clEnqueueWriteImage(cl_command_queue d_q, cl_mem d_mem, cl_bool blocking,
>> > ptr, {}, src_pitch,
>> > region));
>> >
>> > + if (blocking)
>> > + hev().wait();
>> > +
>> > ret_object(rd_ev, hev);
>> > return CL_SUCCESS;
>> >
>> > @@ -667,7 +685,11 @@ clEnqueueMapBuffer(cl_command_queue d_q, cl_mem d_mem, cl_bool blocking,
>> >
>> > void *map = mem.resource(q).add_map(q, flags, blocking, obj_origin, region);
>> >
>> > - ret_object(rd_ev, create<hard_event>(q, CL_COMMAND_MAP_BUFFER, deps));
>> > + auto hev = create<hard_event>(q, CL_COMMAND_MAP_BUFFER, deps);
>> > + if (blocking)
>> > + hev().wait();
>> > +
>> > + ret_object(rd_ev, hev);
>> > ret_error(r_errcode, CL_SUCCESS);
>> > return map;
>> >
>> > @@ -695,7 +717,11 @@ clEnqueueMapImage(cl_command_queue d_q, cl_mem d_mem, cl_bool blocking,
>> >
>> > void *map = img.resource(q).add_map(q, flags, blocking, origin, region);
>> >
>> > - ret_object(rd_ev, create<hard_event>(q, CL_COMMAND_MAP_IMAGE, deps));
>> > + auto hev = create<hard_event>(q, CL_COMMAND_MAP_IMAGE, deps);
>> > + if (blocking)
>> > + hev().wait();
>> > +
>> > + ret_object(rd_ev, hev);
>> > ret_error(r_errcode, CL_SUCCESS);
>> > return map;
>> >
>> > --
>> > 2.13.3
>>
>> _______________________________________________
>> mesa-dev mailing list
>> mesa-dev at lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
> --
> Jan Vesely <jan.vesely at rutgers.edu>
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