Nick Piggin <nickpiggin@xxxxxxxxxxxx> 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
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.
> 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:-/
(2) You lose the advantage of being able to process what you've got whilst the
disk is fetching stuff in the background.
David
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