On 1/17/24 11:43 AM, Jens Axboe wrote: > Certainly slower. Now let's try and have the scheduler place the same 4 > threads where it sees fit: > > IOPS=1.56M, BW=759MiB/s, IOS/call=32/31 > > Yikes! That's still substantially more than 200K IOPS even with heavy > contention, let's take a look at the profile: > > - 70.63% io_uring [kernel.kallsyms] [k] queued_spin_lock_slowpath > - submitter_uring_fn > - entry_SYSCALL_64 > - do_syscall_64 > - __se_sys_io_uring_enter > - 70.62% io_submit_sqes > blk_finish_plug > __blk_flush_plug > - blk_mq_flush_plug_list > - 69.65% blk_mq_run_hw_queue > blk_mq_sched_dispatch_requests > - __blk_mq_sched_dispatch_requests > + 60.61% dd_dispatch_request > + 8.98% blk_mq_dispatch_rq_list > + 0.98% dd_insert_requests > > which is exactly as expected, we're spending 70% of the CPU cycles > banging on dd->lock. Case in point, I spent 10 min hacking up some smarts on the insertion and dispatch side, and then we get: IOPS=2.54M, BW=1240MiB/s, IOS/call=32/32 or about a 63% improvement when running the _exact same thing_. Looking at profiles: - 13.71% io_uring [kernel.kallsyms] [k] queued_spin_lock_slowpath reducing the > 70% of locking contention down to ~14%. No change in data structures, just an ugly hack that: - Serializes dispatch, no point having someone hammer on dd->lock for dispatch when already running - Serialize insertions, punt to one of N buckets if insertion is already busy. Current insertion will notice someone else did that, and will prune the buckets and re-run insertion. And while I seriously doubt that my quick hack is 100% fool proof, it works as a proof of concept. If we can get that kind of reduction with minimal effort, well... -- Jens Axboe
