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

Matthew Wilcox
2020-07-05T03:12:20Z

On Sun, Jul 05, 2020 at 04:46:04AM +0200, Jan Ziak wrote:
> On Sun, Jul 5, 2020 at 4:16 AM Matthew Wilcox <willy@infradead.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.

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.