From: Greg KH <gregkh@linuxfoundation.org>
To: Vito Caputo <vcaputo@pengaru.com>
Cc: Matthew Wilcox <willy@infradead.org>,
Jan Ziak <0xe2.0x9a.0x9b@gmail.com>,
linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
linux-man@vger.kernel.org, mtk.manpages@gmail.com,
shuah@kernel.org, viro@zeniv.linux.org.uk
Subject: Re: [PATCH 0/3] readfile(2): a new syscall to make open/read/close faster
Date: Sun, 5 Jul 2020 13:44:54 +0200 [thread overview]
Message-ID: <20200705114454.GB1224775@kroah.com> (raw)
In-Reply-To: <20200705080714.76m64pwwpvlzji2v@shells.gnugeneration.com>
On Sun, Jul 05, 2020 at 01:07:14AM -0700, Vito Caputo wrote:
> 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.
That's your use case, but many other use cases are just "read a bunch of
sysfs files in one shot". Examples of that are tools that monitor
uevents and lots of hardware-information gathering tools.
Also not all tools sem to be as smart as you think they are, look at
util-linux for loads of the "open/read/close" lots of files pattern. I
had a half-baked patch to convert it to use readfile which I need to
polish off and post with the next series to show how this can be used to
both make userspace simpler as well as use less cpu time.
> 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.
Is psutils using this type of interface, or do they constantly open
different files?
What about fun tools like bashtop:
https://github.com/aristocratos/bashtop.git
which thankfully now relies on python's psutil package to parse proc in
semi-sane ways, but that package does loads of constant open/read/close
of proc files all the time from what I can tell.
And lots of people rely on python's psutil, right?
> 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.
What "good" monitoring programs do you suggest follow the pattern you
recommend?
thanks,
greg k-h
next prev parent reply other threads:[~2020-07-05 11:44 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-05 2:06 [PATCH 0/3] readfile(2): a new syscall to make open/read/close faster Jan Ziak
2020-07-05 2:16 ` Matthew Wilcox
2020-07-05 2:46 ` Jan Ziak
2020-07-05 3:12 ` Matthew Wilcox
2020-07-05 3:18 ` Jan Ziak
2020-07-05 3:27 ` Matthew Wilcox
2020-07-05 4:09 ` Jan Ziak
2020-07-05 11:58 ` Greg KH
2020-07-06 6:07 ` Jan Ziak
2020-07-06 11:11 ` Matthew Wilcox
2020-07-06 11:18 ` Greg KH
2020-07-05 8:07 ` Vito Caputo
2020-07-05 11:44 ` Greg KH [this message]
2020-07-05 20:34 ` Vito Caputo
2020-07-05 6:32 ` Andreas Dilger
2020-07-05 7:25 ` Jan Ziak
2020-07-05 12:00 ` Greg KH
2020-07-05 11:50 ` Greg KH
2020-07-14 6:51 ` Pavel Machek
2020-07-14 8:07 ` Miklos Szeredi
2020-07-14 11:34 ` Pavel Begunkov
2020-07-14 11:55 ` Miklos Szeredi
2020-07-15 8:31 ` Pavel Begunkov
2020-07-15 8:41 ` Miklos Szeredi
2020-07-15 8:49 ` Pavel Begunkov
2020-07-15 9:00 ` Pavel Begunkov
2020-07-15 11:17 ` Miklos Szeredi
-- strict thread matches above, loose matches on Subject: below --
2020-07-04 14:02 Greg Kroah-Hartman
2020-07-04 19:30 ` Al Viro
2020-07-05 11:47 ` Greg Kroah-Hartman
2020-07-06 17:25 ` Dave Martin
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