From: Jens Axboe > Sent: 03 May 2021 19:05 > > On 5/3/21 12:02 PM, Matthew Wilcox wrote: > > On Mon, May 03, 2021 at 11:57:08AM -0600, Jens Axboe wrote: > >> On 5/3/21 10:12 AM, David Laight wrote: > >>> From: Jens Axboe > >>>> Sent: 03 May 2021 15:58 > >>>> > >>>> Had a report on writing to eventfd with io_uring is slower than it > >>>> should be, and it's the usual case of if a file type doesn't support > >>>> ->write_iter(), then io_uring cannot rely on IOCB_NOWAIT being honored > >>>> alongside O_NONBLOCK for whether or not this is a non-blocking write > >>>> attempt. That means io_uring will punt the operation to an io thread, > >>>> which will slow us down unnecessarily. > >>>> > >>>> Convert eventfd to using fops->write_iter() instead of fops->write(). > >>> > >>> Won't this have a measurable performance degradation on normal > >>> code that does write(event_fd, &one, 4); > >> > >> If ->write_iter() or ->read_iter() is much slower than the non-iov > >> versions, then I think we have generic issues that should be solved. > > > > We do! > > > > https://lore.kernel.org/linux-fsdevel/20210107151125.GB5270@xxxxxxxxxxxxxxxxxxxx/ > > is one thread on it. There have been others. > > But then we really must get that fixed, imho ->read() and ->write() > should go away, and if the iter variants are 10% slower, then that should > get fixed up. I think there are two separate issues. (Although I've not looked in detail into the really bad cases.) 1) I suspect some of the fs code is using entirely different paths for the 'single fragment' and 'iter' variants. 2) For trivial drivers the cost of setting up the iov_iter[] and then iterating it becomes significant (or at least measurable). I haven't tried to undo the morass of #defines in the iter code. But I suspect they could be optimised for the common case of copying an entire single-fragment to/from userspace in one call. Not related to this code path, but I've some patches that give a few % speedup for writev() to /dev/null. That is all about copying the iov[] from user - it doesn't get 'iterated'. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
