Re: How git affects kernel.org performance

From: Linus Torvalds <torvalds@osdl.org>
Date: Jan 7, 2007, 19:35 UTC
Message-ID: <Pine.LNX.4.64.0701071132450.3661@woody.osdl.org>
In-Reply-To: <9e4733910701071126r7931042eldfb73060792f4f41@mail.gmail.com>

On Sun, 7 Jan 2007, Jon Smirl wrote:

Show 11 quoted lines

> > 
> >  - proper read-ahead. Right now, even if the directory is totally
> >    contiguous on disk (just remove the thing that writes data to the
> >    files, so that you'll have empty files instead of 8kB files), I think
> >    we do those reads totally synchronously if the filesystem was mounted
> >    with directory hashing enabled.
> 
> What's the status on the Adaptive Read-ahead patch from Wu Fengguang
> <wfg@mail.ustc.edu.cn> ? That patch really helped with read ahead
> problems I was having with mmap. It was in mm forever and I've lost
> track of it.

Won't help. ext3 does NO readahead at all. It doesn't use the general VFS helper routines to read data (because it doesn't use the page cache), it just does the raw buffer-head IO directly.

(In the non-indexed case, it does do some read-ahead, and it uses the generic routines for it, but because it does everything by physical address, even the generic routines will decide that it's just doing random reading if the directory isn't physically contiguous - and stop reading ahead).

(I may have missed some case where it does do read-ahead in the index routines, so don't take my word as being unquestionably true. I'm _fairly_ sure, but..)

			Linus