Re: [PATCH] Enable index-pack threading in msysgit.

On Thu, Mar 20, 2014 at 4:35 AM, Stefan Zager [off-list ref] wrote:

This adds a Windows implementation of pread.  Note that it is NOT
safe to intersperse calls to read() and pread() on a file
descriptor.  According to the ReadFile spec, using the 'overlapped'
argument should not affect the implicit position pointer of the
descriptor.  Experiments have shown that this is, in fact, a lie.

To accomodate that fact, this change also incorporates:

http://article.gmane.org/gmane.comp.version-control.git/196042

... which gives each index-pack thread its own file descriptor.

If the problem is mixing read() and pread() then perhaps it's enough to do

output_fd = dup(output_fd);

after pack_fd is set in open_pack_file(), to make sure that
fixup_pack_header_footer() has its own file handle. If that works, we
don't need one pack_fd per thread.

compat/mmap.c uses pread() and its bad interaction with read() could
turn it into a nightmare. Fortunately Windows (except Cygwin) does not
use this implementation. Not sure if we should make a note about this.

It makes me wonder if sliding mmap window (like we do for pack access
in sha1_file.c) would be better than pread(). index-pack used to do
mmap() [1] in the past with poor performance but I don't think sliding
window was mentioned.

[1] http://thread.gmane.org/gmane.comp.version-control.git/34741/focus=34832

What about the "ifeq ($(uname_S),Windows)" section? I think MSVC and
MinGW builds share a lot of code.
-- 
Duy