Re: statfs() / statvfs() syscall ballsup...

[Joel Becker]

On Fri, Oct 10, 2003 at 10:40:40AM -0700, Linus Torvalds wrote:
> Actually, the kernel has a "readahead(fd, offset, size)" system call that
> will start asynchronous read-ahead on any mapping. After that, just
> touching the page will obviously map in and synchronize the result.

	Ok, a quick peruse of sys_readahead() seems to say that it
doesn't check for existing uptodate()ness.  That would be interesting.
I could have missed it, though.  

> I don't think anybody uses it, and the interface may be broken, but it was
> literally 20 lines of code, and I had a trivial test program that
> populated the cache for a directory structure really quickly using it.

	The problem we have with msync() and friends is not 'quick
population', it's "page is in the page cache already; another node
writes to the storage; must mark page as !uptodate so as to force a
re-read from disk".  I can't find where sys_readahead() checks for
uptodate, so perhaps calling sys_readahead() on a range always causes
I/O.  Correct me if I missed it.

> For example, things we can do, but don't, partly because of interface 
> issues and because there is no point in doing it if people wouldn't use 
> it:

	Lots of interesting stuff snipped.  This discussion has me
thinking, knowing now that there's possibility to move to a more optimal
interface.

Joel

-- 

Life's Little Instruction Book #464

	"Don't miss the magic of the moment by focusing on what's
	 to come."

Joel Becker
Senior Member of Technical Staff
Oracle Corporation
E-mail: joel.becker@xxxxxxxxxx
Phone: (650) 506-8127
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