Re: [PATCH 2/9]: Reduce Log I/O latency

[Andi Kleen]

On Fri, Nov 23, 2007 at 03:03:29PM +1100, David Chinner wrote:
> On Fri, Nov 23, 2007 at 03:53:17AM +0100, Andi Kleen wrote:
> > On Fri, Nov 23, 2007 at 12:15:39AM +1100, David Chinner wrote:
> > > On Thu, Nov 22, 2007 at 01:06:11PM +0100, Andi Kleen wrote:
> > > > > FWIW from a "real time" database POV this seems to make sense to me...
> > > > > in fact, we probably rely on filesystem metadata way too much
> > > > > (historically it's just "worked".... although we do seem to get issues
> > > > > on ext3).
> > > > 
> > > > For that case you really would need priority inheritance: any metadata
> > > > IO on behalf or blocking a process needs to use the process' block IO 
> > > > priority.
> > > 
> > > How do you do that when the processes are blocking on semaphores,
> > > mutexes or rw-semaphores in the fileysystem three layers removed from
> > > the I/O in progress?
> > 
> > [...] I didn't say it was easy (or rather explicitely said it would be tricky).
> > Probably it would be possible to fold it somehow into rt mutexes PI,
> > but it's not easy and semaphores would need to be handled too.
> > 
> > Just my point was to solve the metadata RT problem unconditionally increasing
> > the priority is a bad idea and not really a replacement to a "full"
> > solution. Short term a user can just increase the priority of all the XFS 
> > threads anyways.
> 
> The point is that it's not actually a thread-based problem - the priority
> can't be inherited via the traditional mutex-like manner. There is no
> connection between a thread and an I/o it has already issued and so you
> can't transfer a priority from a blocked thread to an issued-but-blocked
> i/o....

It could be handled in theory similar to standard CPU priority inheritance -- \
keep track of IO priority of all threads you block and boost your IO priority
always to that level. But it would be probably not very easy to do.


-Andi
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