Re: [FFmpeg-devel] [PATCH] avcodec/amfenc: Fix for windows imprecise sleep

[Zhao Zhili <quinkblack@foxmail.com>]

> 在 2023年10月17日，上午5:24，Mark Thompson <sw@jkqxz.net> 写道：
> 
> On 16/10/2023 10:13, Evgeny Pavlov wrote:
>> This commit reduces the sleep time on Windows to improve AMF encoding
>> performance on low resolution input videos.
>> This fix is for Windows only, because sleep() function isn't
>> very accurate on Windows OS.
>> Fix for issue #10622
>> Signed-off-by: Evgeny Pavlov <lucenticus@gmail.com>
>> ---
>>  libavcodec/amfenc.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>> diff --git a/libavcodec/amfenc.c b/libavcodec/amfenc.c
>> index 061859f85c..0c95465d6e 100644
>> --- a/libavcodec/amfenc.c
>> +++ b/libavcodec/amfenc.c
>> @@ -770,7 +770,11 @@ int ff_amf_receive_packet(AVCodecContext *avctx, AVPacket *avpkt)
>>          if (query_output_data_flag == 0) {
>>              if (res_resubmit == AMF_INPUT_FULL || ctx->delayed_drain || (ctx->eof && res_query != AMF_EOF) || (ctx->hwsurfaces_in_queue >= ctx->hwsurfaces_in_queue_max)) {
>>                  block_and_wait = 1;
>> +#ifdef _WIN32
>> +                av_usleep(0); //Sleep() is not precise on Windows OS.
>> +#else
>>                  av_usleep(1000);
>> +#endif
>>              }
>>          }
>>      } while (block_and_wait);
> 
> Wasting lots of power by spinning on a CPU core does not seem like a good answer to this problem.  (I mean, presumably that is why Windows isn't honouring your request for a short sleep, because it wants timers to have larger gaps to avoid wasting power.)

If av_usleep is implemented via Sleep like current case, sleep 0 means yield current thread, so it’s not busy wait in normal case (but it can be busy wait).

av_usleep(500) may looks better and do the same job by depending 500/1000 = 0.

I agree use sleep without real async is like a bug.

> 
> Why is there a sleep here at all, anyway?  An API for hardware encoding should be providing a way for the caller to wait for an outstanding operation to complete.
> 
> Thanks,
> 
> - Mark
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