Re: Buffer and Page cache coherent? was: Strange IDE performance change in 2.6.1-rc1 (again)

[Mike Fedyk <mfedyk@matchmail.com>]

On Sun, Jan 04, 2004 at 03:32:58PM -0800, Andrew Morton wrote:
> Mike Fedyk <mfedyk@matchmail.com> wrote:
> >
> > On Sun, Jan 04, 2004 at 02:10:30PM -0800, Andrew Morton wrote:
> > > Mike Fedyk <mfedyk@matchmail.com> wrote:
> > > >
> > > > On Sat, Jan 03, 2004 at 02:40:03PM -0800, Andrew Morton wrote:
> > > > > No effort was made to optimise buffered blockdev reads because it is not
> > > > > very important and my main interest was in data coherency and filesystem
> > > > > metadata consistency.
> > > > 
> > > > Does that mean that blockdev reads will populate the pagecache in 2.6?
> > > 
> > > They have since 2.4.10.  The pagecache is the only cacheing entity for file
> > > (and blockdev) data.
> > 
> > There was a large thread after 2.4.10 was released about speeding up the
> > boot proces by reading the underlying blockdev of the root partition in
> > block order.
> > 
> > Unfortunately at the time reading the files through the pagecache would
> > cause a second read of the data even if it was already buffered.  I don't
> > remember the exact details.
> 
> The pagecache is a cache-per-inode.  So the cache for a regular file is not
> coherent with the cache for /dev/hda1 is not coherent with the cache for
> /dev/hda.

That's what I remember from the old thread.  Thanks.

Duffers are attached to a page, and blockdev reads will not save
pagecache reads.

So in what way is the buffer cache coherent with the pagecache?

> > Are you saying this is now resolved?  And the above optimization will work?
> 
> It will not.  And I doubt if it will make much difference anyway.  I once
> wrote a gizmo which a) generated tables describing pagecache contents
> immediately after bootup and b) used that info to prepopulate pagecache
> with an optimised seek pattern after boot.  It was only worth 10-15%.  One
> would need an intermediate step which relaid-out the relevant files to get
> useful speedups.

Any progress on that pagecache coherent block relocation patch you had for
ext3? :)