Re: returning non-ram via ->nopage, was Re: [patch] mspec driver for 2.6.12-rc2-mm3

[Jeff Garzik <jgarzik@pobox.com>]

Jes Sorensen wrote:
> > > > > > "Christoph" == Christoph Hellwig <hch@infradead.org> writes:
>
>
> Christoph> http://marc.theaimsgroup.com/?l=linux-kernel&m=111416930927092&w=2),
> Christoph> which has a nopage routine that calls remap_pfn_range from
> Christoph> ->nopage for uncached memory that's not part of the mem
> Christoph> map.  Because ->nopage wants to return a struct page * he's
> Christoph> allocating a normal kernel page and actually returns that
> Christoph> one - to get the page he wants into the pagetables his does
> Christoph> all the pagetable manipulation himself before (See the
> Christoph> glory details of pagetable walks and modification inside a
> Christoph> driver in the patch above).
>
> Christoph> I don't think these hacks are acceptable for a driver,
> Christoph> especially as the problem can easily be solved by calling
> Christoph> remap_pfn_range in ->mmap - except SGI also wants node
> Christoph> locality..
>
> Christoph,
>
> Let me try and provide some more background then.
>
> Simply doing remap_pfn_range in the mmap call doesn't work for large
> systems.
>
> Take the example of a 2048 CPU system (512 CPUs per partition/machine
> - each machine running it's own OS) running an MPI application
> across all 2048 CPUs using cross coherency domain traffic.
>
> A standard application will allocate 56 DDQs per thread (the DDQs are
> used for synchronization and allocated through the mspec driver) which
> translates to having 126976 uncached cache lines reserved or 992 pages
> per worker thread. The controlling thread on each partition will mmap
> the entire DDQ space up front and then fork off the workers who will
> then go and touch their pages. With the current approach by the driver
> this means that if you have two threads per node you will end up with
> ~32MB of uncached memory allocated per node.
>
> Alternatively doing this at mmap time having 512 worker threads per
> partition, the result is ~8GB (992 * 16K * 512) of uncached memory all
> allocated by the master thread on each machine.
>
> A typical system configuration is 4GB or 8GB of RAM per node. This
> means that by using the remap_pfn_range at mmap time approach and the
> kernel's standard overhead you end up completely starving the first
> couple of nodes of memory on each partition.
>
> Combine this with the effect of all synchronization traffic hitting
> the same node, you effectively end up with 512 CPUs all constantly
> hammering the same memory controller to death.
>
> FWIW, an initial implementation of the driver was done by someone
> within SGI, prior to me having anything to do with it. It was using
> the remap_pfn_range at mmap time approach and it was noticed then that
> 16 worker threads was pretty much enough to overwhelm a node.
>
> Having the page allocations and drop ins on a first touch basis is
> consistent with what is done for cached memory and seems a pretty
> reasonable approach to me. Sure it isn't particularly pretty to use
> the ->nopage approach, nobody disagrees with you there, but what is
> the alternative?

I don't see anything wrong with a ->nopage approach.

At Linus's suggestion, I used ->nopage in the implementation of 
sound/oss/via82cxxx_audio.c.

	Jeff



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