On Sun, Jul 05, 2020 at 04:27:32AM +0100, Matthew Wilcox wrote: > On Sun, Jul 05, 2020 at 05:18:58AM +0200, Jan Ziak wrote: > > On Sun, Jul 5, 2020 at 5:12 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > > > You should probably take a look at io_uring. That has the level of > > > complexity of this proposal and supports open/read/close along with many > > > other opcodes. > > > > Then glibc can implement readfile using io_uring and there is no need > > for a new single-file readfile syscall. > > It could, sure. But there's also a value in having a simple interface > to accomplish a simple task. Your proposed API added a very complex > interface to satisfy needs that clearly aren't part of the problem space > that Greg is looking to address. I disagree re: "aren't part of the problem space". Reading small files from procfs was specifically called out in the rationale for the syscall. In my experience you're rarely monitoring a single proc file in any situation where you care about the syscall overhead. You're monitoring many of them, and any serious effort to do this efficiently in a repeatedly sampled situation has cached the open fds and already uses pread() to simply restart from 0 on every sample and not repeatedly pay for the name lookup. Basically anything optimally using the existing interfaces for sampling proc files needs a way to read multiple open file descriptors in a single syscall to move the needle. This syscall doesn't provide that. It doesn't really give any advantage over what we can achieve already. It seems basically pointless to me, from a monitoring proc files perspective. Regards, Vito Caputo
