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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Tue, 14 Jul 2020 13:55:50 +0200
Message-ID: <CAJfpegvroouw5ndHv+395w5PP1c+pUyp=-T8qhhvSnFbhbRehg@mail.gmail.com>
Subject: Re: [PATCH 0/3] readfile(2): a new syscall to make open/read/close faster
To:     Pavel Begunkov <asml.silence@gmail.com>
Cc:     Pavel Machek <pavel@denx.de>, Greg KH <gregkh@linuxfoundation.org>,
        Jan Ziak <0xe2.0x9a.0x9b@gmail.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        linux-man <linux-man@vger.kernel.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>, shuah@kernel.org,
        Al Viro <viro@zeniv.linux.org.uk>, io-uring@vger.kernel.org
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On Tue, Jul 14, 2020 at 1:36 PM Pavel Begunkov <asml.silence@gmail.com> wrote:
>
> On 14/07/2020 11:07, Miklos Szeredi wrote:
> > On Tue, Jul 14, 2020 at 8:51 AM Pavel Machek <pavel@denx.de> wrote:
> >>
> >> Hi!
> >>
> >>>> At first, I thought that the proposed system call is capable of
> >>>> reading *multiple* small files using a single system call - which
> >>>> would help increase HDD/SSD queue utilization and increase IOPS (I/O
> >>>> operations per second) - but that isn't the case and the proposed
> >>>> system call can read just a single file.
> >>>
> >>> If you want to do this for multple files, use io_ring, that's what it
> >>> was designed for.  I think Jens was going to be adding support for the
> >>> open/read/close pattern to it as well, after some other more pressing
> >>> features/fixes were finished.
> >>
> >> What about... just using io_uring for single file, too? I'm pretty
> >> sure it can be wrapped in a library that is simple to use, avoiding
> >> need for new syscall.
> >
> > Just wondering:  is there a plan to add strace support to io_uring?
> > And I don't just mean the syscalls associated with io_uring, but
> > tracing the ring itself.
>
> What kind of support do you mean? io_uring is asynchronous in nature
> with all intrinsic tracing/debugging/etc. problems of such APIs.
> And there are a lot of handy trace points, are those not enough?
>
> Though, this can be an interesting project to rethink how async
> APIs are worked with.

Yeah, it's an interesting problem.  The uring has the same events, as
far as I understand, that are recorded in a multithreaded strace
output (syscall entry, syscall exit); nothing more is needed.

I do think this needs to be integrated into strace(1), otherwise the
usefulness of that tool (which I think is *very* high) would go down
drastically as io_uring usage goes up.

Thanks,
Miklos