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

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Date:   Sun, 5 Jul 2020 04:46:04 +0200
From:   Jan Ziak <0xe2.0x9a.0x9b@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     gregkh@...uxfoundation.org, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-man@...r.kernel.org,
        mtk.manpages@...il.com, shuah@...nel.org, viro@...iv.linux.org.uk
Subject: Re: [PATCH 0/3] readfile(2): a new syscall to make open/read/close faster

On Sun, Jul 5, 2020 at 4:16 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Sun, Jul 05, 2020 at 04:06:22AM +0200, Jan Ziak wrote:
> > Hello
> >
> > 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.
> >
> > Without the ability to read multiple small files using a single system
> > call, it is impossible to increase IOPS (unless an application is
> > using multiple reader threads or somehow instructs the kernel to
> > prefetch multiple files into memory).
>
> What API would you use for this?
>
> ssize_t readfiles(int dfd, char **files, void **bufs, size_t *lens);
>
> I pretty much hate this interface, so I hope you have something better
> in mind.

I am proposing the following:

struct readfile_t {
  int dirfd;
  const char *pathname;
  void *buf;
  size_t count;
  int flags;
  ssize_t retval; // set by kernel
  int reserved; // not used by kernel
};

int readfiles(struct readfile_t *requests, size_t count);

Returns zero if all requests succeeded, otherwise the returned value
is non-zero (glibc wrapper: -1) and user-space is expected to check
which requests have succeeded and which have failed. retval in
readfile_t is set to what the single-file readfile syscall would
return if it was called with the contents of the corresponding
readfile_t struct.

The glibc library wrapper of this system call is expected to store the
errno in the "reserved" field. Thus, a programmer using glibc sees:

struct readfile_t {
  int dirfd;
  const char *pathname;
  void *buf;
  size_t count;
  int flags;
  ssize_t retval; // set by glibc (-1 on error)
  int errno; // set by glibc if retval is -1
};

retval and errno in glibc's readfile_t are set to what the single-file
glibc readfile would return (retval) and set (errno) if it was called
with the contents of the corresponding readfile_t struct. In case of
an error, glibc will pick one readfile_t which failed (such as: the
1st failed one) and use it to set glibc's errno.