Re: [PATCH 2/7] Switch git_mmap to use pread.

From: Shawn Pearce <spearce@spearce.org>
Date: Dec 24, 2006, 20:13 UTC
Message-ID: <20061224201304.GA631@spearce.org>
In-Reply-To: <Pine.LNX.4.63.0612241407250.19693@wbgn013.biozentrum.uni-wuerzburg.de>

Johannes Schindelin <Johannes.Schindelin@gmx.de> wrote:

Show 13 quoted lines

> On Sun, 24 Dec 2006, Shawn O. Pearce wrote:
> > Now that Git depends on pread in index-pack its safe to say we can
> > also depend on it within the git_mmap emulation we activate when
> > NO_MMAP is set.  On most systems pread should be slightly faster
> > than an lseek/read/lseek sequence as its one system call vs. three
> > system calls.
> 
> I don't think it matters much. The _only_ platform we really use NO_MMAP 
> (other than for testing) is Windows, and AFAICT it does not have pread(), 
> so it is emulated by lseek/read/lseek anyway.
> 
> But it's a cleanup, and it deletes more lines than it adds, so Ack from 
> me.

Right - that's why I did it. I was already in there doing other work and discovered that we had a lot of lines just to avoid using pread. Initially that may have been because we were trying to avoid using it, but now that one of our more important tools (index-pack) depends on it... ;-)

I'm actually thinking of doing a NO_PACK_MMAP, especially for systems like Mac OS X where the mmap() of small chunks appears to perform so poorly. Setting NO_PACK_MMAP and just using smaller windows (1 MiB) might be worthwhile. Though in that case we may just want to compile with NO_MMAP.

-- 
Shawn.