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X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: 5XU+W6yuCRRgMH9CTAFI1E1sFiFqBAqqYDasvV3hB7QNUjBKO+gfjMUmzZe66N+jjAEYBL6P6a59pDThnhnvhQ3DeZFOSxN1UOcjX9tg4mo= X-MS-Exchange-Transport-CrossTenantHeadersStamped: BLAPR10MB4868 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1039,Hydra:6.0.680,FMLib:17.12.28.16 definitions=2024-08-12_12,2024-08-12_02,2024-05-17_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 adultscore=0 mlxscore=0 bulkscore=0 phishscore=0 malwarescore=0 suspectscore=0 spamscore=0 mlxlogscore=968 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2407110000 definitions=main-2408130039 X-Proofpoint-GUID: sZSCQRjZb1i9SIp8L4wHecDdEXibjnW- X-Proofpoint-ORIG-GUID: sZSCQRjZb1i9SIp8L4wHecDdEXibjnW- Linus Torvalds writes: > On Mon, 12 Aug 2024 at 10:33, Shrikanth Hegde wrote: >> >> top 3 callstacks of __schedule collected with bpftrace. >> >> preempt=none preempt=full >> >> __schedule+12 |@[ >> schedule+64 | __schedule+12 >> interrupt_exit_user_prepare_main+600 | preempt_schedule+84 >> interrupt_exit_user_prepare+88 | _raw_spin_unlock_irqrestore+124 >> interrupt_return_srr_user+8 | __wake_up_sync_key+108 >> , hackbench]: 482228 | pipe_write+1772 >> @[ | vfs_write+1052 >> __schedule+12 | ksys_write+248 >> schedule+64 | system_call_exception+296 >> pipe_write+1452 | system_call_vectored_common+348 >> vfs_write+940 |, hackbench]: 538591 >> ksys_write+248 |@[ >> system_call_exception+292 | __schedule+12 >> system_call_vectored_common+348 | schedule+76 >> , hackbench]: 1427161 | schedule_preempt_disabled+52 >> @[ | __mutex_lock.constprop.0+1748 >> __schedule+12 | pipe_write+132 >> schedule+64 | vfs_write+1052 >> interrupt_exit_user_prepare_main+600 | ksys_write+248 >> syscall_exit_prepare+336 | system_call_exception+296 >> system_call_vectored_common+360 | system_call_vectored_common+348 >> , hackbench]: 8151309 |, hackbench]: 5388301 >> @[ |@[ >> __schedule+12 | __schedule+12 >> schedule+64 | schedule+76 >> pipe_read+1100 | pipe_read+1100 >> vfs_read+716 | vfs_read+716 >> ksys_read+252 | ksys_read+252 >> system_call_exception+292 | system_call_exception+296 >> system_call_vectored_common+348 | system_call_vectored_common+348 >> , hackbench]: 18132753 |, hackbench]: 64424110 >> > > So the pipe performance is very sensitive, partly because the pipe > overhead is normally very low. > > So we've seen it in lots of benchmarks where the benchmark then gets > wildly different results depending on whether you get the goo "optimal > pattern". > > And I think your "preempt=none" pattern is the one you really want, > where all the pipe IO scheduling is basically done at exactly the > (optimized) pipe points, ie where the writer blocks because there is > no room (if it's a throughput benchmark), and the reader blocks > because there is no data (for the ping-pong or pipe ring latency > benchmarks). > > And then when you get that "perfect" behavior, you typically also get > the best performance when all readers and all writers are on the same > CPU, so you get no unnecessary cache ping-pong either. > > And that's a *very* typical pipe benchmark, where there are no costs > to generating the pipe data and no costs involved with consuming it > (ie the actual data isn't really *used* by the benchmark). > > In real (non-benchmark) loads, you typically want to spread the > consumer and producer apart on different CPUs, so that the real load > then uses multiple CPUs on the data. But the benchmark case - having > no real data load - likes the "stay on the same CPU" thing. > > Your traces for "preempt=none" very much look like that "both reader > and writer sleep synchronously" case, which is the optimal benchmark > case. > > And then with "preempt=full", you see that "oh damn, reader and writer > actually hit the pipe mutex contention, because they are presumably > running at the same time on different CPUs, and didn't get into that > nice serial synchronous pattern. So now you not only have that mutex > overhead (which doesn't exist in the reader and writer synchronize), > you also end up with the cost of cache misses *and* the cost of > scheduling on two different CPU's where both of them basically go into > idle while waiting for the other end. Thanks. That was very clarifying. -- ankur