linux-kernel - Re: [PATCH 23/43] CacheFiles: Permit the page lock state to be monitored [ver #46]

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Date:	Fri, 3 Apr 2009 05:03:18 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	David Howells <dhowells@...hat.com>
Cc:	viro@...iv.linux.org.uk, nfsv4@...ux-nfs.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 23/43] CacheFiles: Permit the page lock state to be monitored [ver #46]

On Friday 03 April 2009 04:05:07 David Howells wrote:
> Nick Piggin <nickpiggin@...oo.com.au> wrote:
> 
> > Presumably: at the point where data is needed.
> 
> But the point where the data is needed is where filemap.c is waiting on a
> netfs page.  Maybe the sync_page() aop can deal with it

I was thinking of more like just have some threads submitting the reads
and unlocking the waiters.

 
> There's also the problem of recording and pinning the backing page I'm waiting
> for.  Currently I can do that by hooking the monitor block into the page
> unlock watching list.  If I don't do that, I have to use up yet more memory to
> track those some other way.  It's not impossible, but I'd like to keep memory
> usage down.
> 
> > Or do you actually have numbers showing a problem if you just read the pages
> > then copy them?
> 
> I did, years ago.  It wasn't particularly good, but
> fscache_read_or_alloc_pages() was completely synchronous.

Performance wasn't good? Why?


> > If there is a problem, then why doesn't fscache_read_or_alloc_pages caller
> > do the work itself, then you get as many threads as you have indivisible
> > work units, so completing some part of the request before another wouldn't
> > gain you anything anyway...
> 
> (1) Trond stipulated FS-Cache had to be asynchronous, and it is, as far as I
>     can make it.  I still have to invoke bmap() synchronously, though, to find
>     out whether I have a page in the cache to read:-/

Why does it have to be asynchronous? Seems like an incredible complexity.
->readpage is called only in synchonous contexts, and ->readpages not but
do you even have the netfs filesystem request readahead on the backing
filesystem? (which then presumably tries to do readahead of its own)

 
> (2) You lose the advantage of being able to process what you've got whilst the
>     disk is fetching stuff in the background.

This should happen via readahead on the underlying filesystem, shouldn't
it?
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