Pierre Labat <plabat@xxxxxxxxxx> writes: > Hi, > > Had some time to re-do some testing. > > 1) Pipewire (its wireplumber deamon) set a watch on the children of the directory /dev via inotify. > I removed that (disabled pipewire), but still had the fsnotify > overhead when using aio/io_ring at high IOPS across several threads on > several cores. > > 2) I then noticed that udev set a watch (via inotify) on the files in /dev. > This is due to a rule in /usr/lib/udev/rules.d/60-block.rules > # watch metadata changes, caused by tools closing the device node which was opened for writing > ACTION!="remove", SUBSYSTEM=="block", \ > KERNEL=="loop*|mmcblk*[0-9]|msblk*[0-9]|mspblk*[0-9]|nvme*|sd*|vd*|xvd*|bcache*|cciss*|dasd*|ubd*|ubi*|scm*|pmem*|nbd*|zd*", > \ > OPTIONS+="watch" > I removed "nvme*" from this rule (I am testing on /dev/nvme0n1), then finally the fsnotify overhead disappeared. Interesting. I don't see that behavior. I setup a null block device with the following parameters: modprobe null_blk submit_queues=96 queue_mode=2 gb=350 bs=4096 completion_nsec=0 hw_queue_depth=1024 And I ran the following fio job: --- [global] ioengine=io_uring iodepth=8 direct=1 rw=read filename=/dev/nullb0 cpus_allowed=0-95 cpus_allowed_policy=split size=1g offset_increment=10g [32thread] numjobs=32 --- If there are no watches on /dev or /dev/nullb0, then I see 70-79GiB/s throughput from this job. If I add a watch on /dev/nullb0, there appears to be a small performance hit, but it is within the run-to-run variation. If I instead add a watch to /dev, the throughput drops to ~10GiB/s. So, I think this matches your initial report (the perf top output closely matched yours). Can you run the attached script to verify that nothing is watching /dev when you have udev configured to watch the nvme device, and report back? Run it with the path as the argument, so "inotify-watchers.sh /dev". If there is no watch on /dev, and you still see a performance problem, then we'll need to start investigating that. A good starting point would be details of how you are testing, along with new perf top output. -Jeff > 3) I think there is nothing wrong with Pipewire and udev, they simply want to watch what is going on in /dev. > I don't think they are interested in (and it is not the goal/charter > of fsnotify) quantifying millions of read/write accesses/sec to a file > they watch. There are other tools for that, that are optimized for > that task. > > I think to avoid the overhead, the fsnotify subsystem should be > refined to factor high frequency read/write file access. > Or piece of code (like aio/io_uring) doing high frequency fsnotify should do the factoring themselves. > Or the user should be given a way to turn off fsnotify calls for read/write on specific file. > Now, the only way to work around the cpu overhead without hacking, is > to disable services watching /dev. That means people can't use these > services anymore. Doesn't seem right. > > Regards, > > Pierre > > >> -----Original Message----- >> From: Pierre Labat >> Sent: Monday, August 14, 2023 9:31 AM >> To: Jeff Moyer <jmoyer@xxxxxxxxxx> >> Cc: Jens Axboe <axboe@xxxxxxxxx>; 'io-uring@xxxxxxxxxxxxxxx' <io- >> uring@xxxxxxxxxxxxxxx> >> Subject: RE: [EXT] Re: FYI, fsnotify contention with aio and io_uring. >> >> Hi Jeff, >> >> Indeed, by default, in my configuration, pipewire is running. >> When I can re-test, I'll disabled it and see if that remove the problem. >> Thanks for the hint! >> >> Pierre >> >> > -----Original Message----- >> > From: Jeff Moyer <jmoyer@xxxxxxxxxx> >> > Sent: Wednesday, August 9, 2023 10:15 AM >> > To: Pierre Labat <plabat@xxxxxxxxxx> >> > Cc: Jens Axboe <axboe@xxxxxxxxx>; 'io-uring@xxxxxxxxxxxxxxx' <io- >> > uring@xxxxxxxxxxxxxxx> >> > Subject: Re: [EXT] Re: FYI, fsnotify contention with aio and io_uring. >> > >> > CAUTION: EXTERNAL EMAIL. Do not click links or open attachments unless >> > you recognize the sender and were expecting this message. >> > >> > >> > Pierre Labat <plabat@xxxxxxxxxx> writes: >> > >> > > Micron Confidential >> > > >> > > Hi Jeff and Jens, >> > > >> > > About "FAN_MODIFY fsnotify watch set on /dev". >> > > >> > > Was using Fedora34 distro (with 6.3.9 kernel), and fio. Without any >> > particular/specific setting. >> > > I tried to see what could watch /dev but failed at that. >> > > I used the inotify-info tool, but that display watchers using the >> > > inotify interface. And nothing was watching /dev via inotify. >> > > Need to figure out how to do the same but for the fanotify interface. >> > > I'll look at it again and let you know. >> > >> > You wouldn't happen to be running pipewire, would you? >> > >> > https://urldefense.com/v3/__https://gitlab.freedesktop.org/pipewire/pi >> > pewir >> > e/- >> > /commit/88f0dbd6fcd0a412fc4bece22afdc3ba0151e4cf__;!!KZTdOCjhgt4hgw!6E >> > 063jj >> > -_XK1NceWzms7DaYacILy4cKmeNVA3xalNwkd0zrYTX-IouUnvJ8bZs-RG3YSdk5XpFoo$ >> > >> > -Jeff >> > >> > > >> > > Regards, >> > > >> > > Pierre >> > > >> > > >> > > >> > > Micron Confidential >> > >> -----Original Message----- >> > >> From: Jens Axboe <axboe@xxxxxxxxx> >> > >> Sent: Tuesday, August 8, 2023 2:41 PM >> > >> To: Jeff Moyer <jmoyer@xxxxxxxxxx>; Pierre Labat >> > >> <plabat@xxxxxxxxxx> >> > >> Cc: 'io-uring@xxxxxxxxxxxxxxx' <io-uring@xxxxxxxxxxxxxxx> >> > >> Subject: [EXT] Re: FYI, fsnotify contention with aio and io_uring. >> > >> >> > >> CAUTION: EXTERNAL EMAIL. Do not click links or open attachments >> > >> unless you recognize the sender and were expecting this message. >> > >> >> > >> >> > >> On 8/7/23 2:11?PM, Jeff Moyer wrote: >> > >> > Hi, Pierre, >> > >> > >> > >> > Pierre Labat <plabat@xxxxxxxxxx> writes: >> > >> > >> > >> >> Hi, >> > >> >> >> > >> >> This is FYI, may be you already knows about that, but in case >> > >> >> you >> > >> don't.... >> > >> >> >> > >> >> I was pushing the limit of the number of nvme read IOPS, the FIO >> > >> >> + the Linux OS can handle. For that, I have something special >> > >> >> under the Linux nvme driver. As a consequence I am not limited >> > >> >> by whatever the NVME SSD max IOPS or IO latency would be. >> > >> >> >> > >> >> As I cranked the number of system cores and FIO jobs doing >> > >> >> direct 4k random read on /dev/nvme0n1, I hit a wall. The IOPS >> > >> >> scaling slows (less than linear) and around 15 FIO jobs on 15 >> > >> >> core threads, the overall IOPS, in fact, goes down as I add more >> > >> >> FIO jobs. For example on a system with 24 cores/48 threads, when >> > >> >> I goes beyond 15 FIO jobs, the overall IOPS starts to go down. >> > >> >> >> > >> >> This happens the same for io_uring and aio. Was using kernel >> > >> >> version >> > >> 6.3.9. Using one namespace (/dev/nvme0n1). >> > >> > >> > >> > [snip] >> > >> > >> > >> >> As you can see 76% of the cpu on the box is sucked up by >> > >> >> lockref_get_not_zero() and lockref_put_return(). Looking at the >> > >> >> code, there is contention when IO_uring call fsnotify_access(). >> > >> > >> > >> > Is there a FAN_MODIFY fsnotify watch set on /dev? If so, it >> > >> > might be a good idea to find out what set it and why. >> > >> >> > >> This would be my guess too, some distros do seem to do that. The >> > >> notification bits scale horribly, nobody should use it for anything >> > >> high performance... >> > >> >> > >> -- >> > >> Jens Axboe
Attachment:
inotify-watchers.sh
Description: Bourne shell script
