Re: [Patch, fortran] PR 56981 Improve unbuffered unformatted performance

[Janne Blomqvist <blomqvist dot janne at gmail dot com>]

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On Fri, Apr 19, 2013 at 1:30 PM, Janne Blomqvist
<blomqvist.janne@gmail.com> wrote:
> Hi,
>
> the attached patch improves the performance for unformatted and
> unbuffered files. Currently unbuffered unformatted really means that
> we don't buffer anything and use the POSIX I/O syscalls directly. With
> the patch, we use the buffer but flush it at the end of each I/O
> statement.
>
> (For formatted I/O we essentially already do this, as the format
> buffer (fbuf) buffers each record).
>
> For the ever important benchmark of writing small (containing a single
> scalar 4 byte value) unformatted sequential records to /dev/null, the
> patch reduces the number of syscalls by a factor of 6, and performance
> improves by more than double.
>
> For trunk, time for the benchmark in the PR:
>
> real    0m0.727s
> user    0m0.272s
> sys     0m0.452s
>
> With the patch:
>
> real    0m0.313s
> user    0m0.220s
> sys     0m0.092s
>
> For comparison, writing to a file where we use buffered I/O:
>
> real    0m0.202s
> user    0m0.180s
> sys     0m0.020s
>
>
> As a semi-unrelated minor improvement, the patch also changes the
> ordering when writing out unformatted sequential record markers.
> Currently we do
>
> write bogus marker
> write record data
> write tail marker
> seek back to before the bogus marker
> write the correct head marker
> seek to the end of the record, behind the tail marker
>
> With the patch we instead do
>
> write bogus marker
> write record data
> seek back to before the bogus marker
> write the correct head marker
> seek to the end of the record data
> write tail marker
>
> With the patch, the slightly shorter seek distances ever-so-slightly
> increase the chance that the seeks will be contained within the buffer
> so we don't have to flush.
>
> Regtested on x86_64-unknown-linux-gnu, Ok for trunk?
>
> 2013-04-19  Janne Blomqvist  <jb@gcc.gnu.org>
>
>     PR fortran/56981
>     * io/transfer.c (next_record_w_unf): First fix head marker, then
>     write tail.
>     (next_record): Call flush_if_unbuffered.
>     * io/unix.c (struct unix_stream): Add field unbuffered.
>     (flush_if_unbuffered): New function.
>     (fd_to_stream): New argument.
>     (open_external): Fix fd_to_stream call.
>     (input_stream): Likewise.
>     (output_stream): Likewise.
>     (error_stream): Likewise.
>     * io/unix.h (flush_if_unbuffered): New prototype.
>
>
> --
> Janne Blomqvist



--
Janne Blomqvist