svn commit: r358152 - head/bin/sh

[Li-Wen Hsu]

On Sat, Feb 22, 2020 at 4:58 AM Antoine Brodin <antoine at freebsd.org> wrote:
>
> On Thu, Feb 20, 2020 at 4:01 AM Hiroki Sato <hrs at freebsd.org> wrote:
> >
> > Author: hrs
> > Date: Thu Feb 20 03:01:27 2020
> > New Revision: 358152
> > URL: https://svnweb.freebsd.org/changeset/base/358152
> >
> > Log:
> >   Improve performance of "read" built-in command when using a seekable
> >   fd.
> >
> >   The read built-in command calls read(2) with a 1-byte buffer because
> >   newline characters need to be detected even on a byte stream which
> >   comes from a non-seekable file descriptor.  Because of this, the
> >   following script calls >6,000 read(2) to show a 6KiB file:
> >
> >    while read IN; do echo "$IN"; done < /COPYRIGHT
> >
> >   When the input byte stream is seekable, it is possible to read a data
> >   block and then reposition the file pointer to where a newline
> >   character found.  This change adds a small buffer to do this and
> >   reduces the number of read(2) calls.
> >
> >   Theoretically, multiple built-in commands reading the same seekable
> >   byte stream in a single pipe chain can share the buffer.  However,
> >   this change just makes a single invocation of the read built-in
> >   allocate a buffer and deallocate it every time for simplicity.
> >   Although this causes read(2) to read the same regions multiple times,
> >   the performance penalty should be small compared to the reduction of
> >   read(2) calls.
> >
> >   Reviewed by:          jilles
> >   MFC after:            1 week
> >   Differential Revision:        https://reviews.freebsd.org/D23747
>
> This seems to be broken on at least i386.
> Please either fix or revert.
>
> Antoine (with hat: portmgr)

Could you provide more detail?  I'm worried because I didn't see
related regression from the recent test results. We may need to add
more test against the breakage you mentioned.

Thanks,
Li-Wen