Re: blkback global resources

[Matt Wilson <msw@amazon.com>]

On Mon, Mar 26, 2012 at 09:20:08AM -0700, Ian Campbell wrote:
> On Mon, 2012-03-26 at 16:56 +0100, Jan Beulich wrote:
> > 
> > Does anyone have an opinion here, in particular regarding the
> > original authors' decision to make this global vs. the apparently
> > made observation (by Daniel Stodden, the author of said patch,
> > who I don't have any current email of to ask directly), but also
> > in the context of multi-page rings, the purpose of which is to
> > allow for larger amounts of in-flight I/O?
> 
> Not really much to say other than we (well, mostly Wei Liu) have a
> similar issue with netback too. That used to have a global pool, then a
> pool-per-worker thread. When Wei added thread-per-vif support he solved
> this by adding a "page pool" which handles allocations. Possibly this
> could grow some sort of fairness etc nobs and be shared with blkback?

I'd definitely welcome an approach in blkback similar to the page pool
changes to netback. Steve Noonan's change to use a spinlock per
blkfront device [1] should show increased contention on the global
request list on the blkback side when multiple devices are attached.

Does netback not also need to handle questions of fairness and
resource starvation?

Matt

[1] http://git.kernel.org/?p=linux/kernel/git/axboe/linux-block.git;a=commitdiff;h=3467811e26660eb46bc655234573d22d6876d5f9