Hi again! On 22/12/2024 11:32, Leon Woestenberg wrote:
You could consider using isolated CPU cores, which should give you much lower latencies, if you have full control over the application logic.
OK, I got around to playing with isolcpus. As a first attempt I added isolcpus=1,2,3 to my kernel commandline and tried stress -m 1 --vm-stride 16 --vm-bytes 512000000 --vm-keep -c 1 and it ran on core 0 maxxing it out. So, all fine and dandy up to this point. Then I went on to run cyclictest on the isolated core: sudo cyclictest -m -p 95 -a 1,2,3 -t 3 But I get enormous latencies: # /dev/cpu_dma_latency set to 0us policy: fifo: loadavg: 2.98 3.08 2.95 3/222 134837 T: 0 (134706) P:95 I:1000 C: 4824 Min: 1 Act: 3 Avg: 8 Max: 518 T: 1 (134707) P:95 I:1500 C: 3215 Min: 1 Act: 1 Avg: 8 Max: 566 T: 2 (134708) P:95 I:2000 C: 2411 Min: 1 Act: 1 Avg: 9 Max: 503 Side note: I get these enormous latencies also without cpu isolation and this particular stress test (albeit with -c 4 and -m 4 - so the stressor running on all cores.) But I would have expected the core isolation to mitigate this somewhat. Here's the really surprising thing though. If I run sudo cyclictest -m -p 95 -a 0,1,2,3 -t 4 then I get these enormous latencies on cores 1,2 and 3 but not on core 0: # /dev/cpu_dma_latency set to 0us policy: fifo: loadavg: 2.50 2.83 2.88 3/223 139478 T: 0 (139313) P:95 I:1000 C: 5881 Min: 2 Act: 3 Avg: 3 Max: 10 T: 1 (139314) P:95 I:1500 C: 3920 Min: 1 Act: 1 Avg: 7 Max: 419 T: 2 (139315) P:95 I:2000 C: 2940 Min: 1 Act: 1 Avg: 7 Max: 480 T: 3 (139316) P:95 I:2500 C: 2352 Min: 1 Act: 1 Avg: 9 Max: 433 And _that_ is really strange from my understanding. What is this raspberry pi 5 doing? O_o 10 us would be an excellent worst case latency ;) And here's another strange thing: The latencies depend a lot on the --vm-stride argument to stress. 16 seems to be the value that maximizes these latencies. IF I go either to higher (32, 64, ...) or lower (8, 4, 2, 1) values the latencies disappear almost completely. Now I am more puzzled than before. Kind regards, FPS
