Re: [PATCH v3 2/3] sha1_file: open window into packfiles with O_CLOEXEC

From: Jeff King <peff@peff.net>
Date: Oct 28, 2016, 07:51 UTC
Message-ID: <20161028075104.la24zydnr3ogb6qv@sigill.intra.peff.net>
In-Reply-To: <CA+55aFwfhFqV74s_O=GucycY9U19ysiACDqX=mK4Gf=eQ0coxQ@mail.gmail.com>

On Thu, Oct 27, 2016 at 03:38:59PM -0700, Linus Torvalds wrote:

Show 14 quoted lines

> On Thu, Oct 27, 2016 at 3:24 AM, Jeff King <peff@peff.net> wrote:
> >
> > +cc Linus as the original author of 144bde78e9 in case there is
> > something subtle I'm missing, but this really just seems like it's
> > an outdated optimization.
> 
> I'd *really* like to keep O_NOATIME if at all possible. It made a huge
> difference on older kernels, and I'm not convinced that relatime
> really fixes it as well as O_NOATIME.
> 
> There are people who don't like relatime. And even if you do have
> relatime enabled, it will update atime once every day, so then this
> makes your filesystem have a storm of nasty inode writebacks if you
> haven't touched that git repo in a while.

The existence of "relatime" is only half the story of its outdatedness.

The other half is packfiles, so that we are paying atime only once per packfile, not once per object (technically once per mmap(), so on a 32-bit system with large packfiles, it would be multiple, depending on your window size).

So I'm not convinced that "storm" is really the right word in a modern context. The atime updates due to object accesses are probably smaller than those from all the other read() calls being done on non-object files (like config, refs, etc).

That being said, if you really care, it's not that much code to keep.

-Peff