Re: [PATCH 0/3] readfile(2): a new syscall to make open/read/close faster

Vito Caputo
2020-07-05T08:14:46Z

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@infradead.org> 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