On Fri, Apr 24, 2015 at 09:30:40AM -0500, Christoph Lameter wrote: > On Thu, 23 Apr 2015, Paul E. McKenney wrote: > > > If by "entire industry" you mean everyone who might want to use hardware > > acceleration, for example, including mechanical computer-aided design, > > I am skeptical. > > The industry designs GPUs with super fast special ram and accellerators > with special ram designed to do fast searches and you think you can demand > page > that stuff in from the main processor? >
Why do you think AMD and NVidia are adding page fault support to their GPU in the first place ? They are not doing this on a whim, they have carefully thought about that. Are you saying you know better than the 2 biggest GPU designer on the planet ? And who do you think is pushing for such thing in the kernel ? Do you think we are working on this on a whim ? Because we woke up one day and thought that it would be cool and that it should be done this way ? Yes if all your GPU do is pagefault it will be disastrous, but is this the usual thing we see on CPU ? No ! Are people complaining about the numerous page fault that happens over a day ? No, the vast majority of user are completely oblivious to page fault. This is how it works on CPU and yes this can work for GPU too. What happens on CPU ? Well CPU can switch to work on a different thread or a different application altogether. The same thing will happen on the GPU. If you have enough jobs, your GPU will be busy and you will never worry about page fault because overall your GPU will deliver the same kind of throughput as if there was no pagefault. It can very well be buried into the overall noise if the ratio of available runnable thread versus page faulting thread is high enough. Which is most of the time the case for the CPU, why would the same assumption not work on the GPU ? Note that i am not dismissing low latency folks, i know they exist, i know they hate page fault and in no way what we propose will make it worse for them. They will be able to keep the same kind of control they cherish but this does not mean you should go on a holy crusade to pretend that other people workload does not exist. They do exist. Page fault is not evil and it has prove usefull to the whole computer industry for CPU. To be sure you are not misinterpretting what we propose, in no way we say we gonna migrate thing on page fault for everyone. We are saying first the device driver decide where thing need to be (system memory or local memory) device driver can get hint/request from userspace for this (as they do today). So no change whatsoever here, people that hand tune things will keep being able to do so. Now we want to add the case where device driver do not get any kind of directive or hint from userspace. So what autonuma is, simply collect informations from the GPU on what is access often and then migrate this transparently (yes this can happen without interruption to GPU). So you are migrating from a memory that has 16GB/s or 32GB/s bandwidth to the device memory that have 500GB/s. This is a valid usecase, they are many people outthere that do not want to learn about hand tuning there application for the GPU but they could nonetheless benefit from it. Cheers, Jérôme -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
