Re: Interacting with coherent memory on external devices

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Apr 23, 2015 at 09:20:55AM -0500, Christoph Lameter wrote:
> On Thu, 23 Apr 2015, Benjamin Herrenschmidt wrote:
> 
> > > There are hooks in glibc where you can replace the memory
> > > management of the apps if you want that.
> >
> > We don't control the app. Let's say we are doing a plugin for libfoo
> > which accelerates "foo" using GPUs.
> 
> There are numerous examples of malloc implementation that can be used for
> apps without modifying the app.

Except that the app might be mapping a file or operating on a big
array in bss instead of (or as well as) using malloc()ed memory.

> > Now some other app we have no control on uses libfoo. So pointers
> > already allocated/mapped, possibly a long time ago, will hit libfoo (or
> > the plugin) and we need GPUs to churn on the data.
> 
> IF the GPU would need to suspend one of its computation thread to wait on
> a mapping to be established on demand or so then it looks like the
> performance of the parallel threads on a GPU will be significantly
> compromised. You would want to do the transfer explicitly in some fashion
> that meshes with the concurrent calculation in the GPU. You do not want
> stalls while GPU number crunching is ongoing.

Yep.  But for throughput-oriented applications, as long as stalls don't
happen very often, this can be OK.

> > The point I'm making is you are arguing against a usage model which has
> > been repeatedly asked for by large amounts of customer (after all that's
> > also why HMM exists).
> 
> I am still not clear what is the use case for this would be. Who is asking
> for this?

Ben and I are.  I have added a use case, which I will send out shortly
with the next version.

							Thanx, Paul

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>