Messages in this thread | | | Date | Mon, 3 Mar 2008 13:15:40 -0600 | From | Jack Steiner <> | Subject | Re: [PATCH] mmu notifiers #v8 |
| |
On Mon, Mar 03, 2008 at 07:45:17PM +0100, Nick Piggin wrote: > On Mon, Mar 03, 2008 at 12:06:05PM -0600, Jack Steiner wrote: > > On Mon, Mar 03, 2008 at 05:59:10PM +0100, Nick Piggin wrote: > > > > Maintaining a long-term reference on a page is a problem. The GRU does not > > > > currently maintain tables to track the pages for which dropins have been done. > > > > > > > > The GRU has a large internal TLB and is designed to reference up to 8PB of > > > > memory. The size of the tables to track this many referenced pages would be > > > > a problem (at best). > > > > > > Is it any worse a problem than the pagetables of the processes which have > > > their virtual memory exported to GRU? AFAIKS, no; it is on the same > > > magnitude of difficulty. So you could do it without introducing any > > > fundamental problem (memory usage might be increased by some constant > > > factor, but I think we can cope with that in order to make the core patch > > > really nice and simple). > > > > Functionally, the GRU is very close to what I would consider to be the > > "standard TLB" model. Dropins and flushs map closely to processor dropins > > and flushes for cpus. The internal structure of the GRU TLB is identical to > > the TLB of existing cpus. Requiring the GRU driver to track dropins with > > long term page references seems to me a deviation from having the basic > > mmuops support a "standard TLB" model. AFAIK, no other processor requires > > this. > > That is because the CPU TLBs have the mmu_gather batching APIs which > avoid the problem. It would be possible to do something similar for > GRU which would involve taking a reference for each page-to-be-invalidated > in invalidate_page, and release them when you invalidate_range. Or else > do some other scheme which makes mmu notifiers work similarly to the > mmu gather API. But not just go an invent something completely different > in the form of this invalidate_begin,clear linux pte,invalidate_end API.
Correct. If the mmu_gather were passed on the mmuops callout and the callout were done at the same point as the tlb_finish_mmu(), the GRU could efficiently work w/o the range invalidates. A range invalidate might still be slightly more efficient but not measureable so. The net difference is not worth the extra complexity of range callouts.
> > > > Tracking TLB dropins (and long term page references) could be done but it > > adds significant complexity and scaling issues. The size of the tables to > > track many TB (to PB) of memory can get large. If the memory is being > > referenced by highly threaded applications, then the problem becomes even > > more complex. Either tables must be replicated per-thread (and require even > > more memory), or the table structure becomes even more complex to deal with > > node locality, cacheline bouncing, etc. > > I don't think it would be that significant in terms of complexity or > scaling. > > For a quick solution, you could stick a radix tree in each of your mmu > notifiers registered (ie. one per mm), which is indexed on virtual address > >> PAGE_SHIFT, and returns the struct page *. Size is no different than > page tables, and locking is pretty scalable. > > After that, I would really like to see whether the numbers justify > larger changes.
I'm still concerned about performance. Each dropin would first have to access an additional data structure that would most likely be non-node-local and non-cache-resident. The net effect would be measurable but not a killer.
I haven't thought about locking requirements for the radix tree. Most accesses would be read-only & updates infrequent. Any chance of an RCU-based radix implementation? Otherwise, don't we add the potential for hot locks/cachelines for threaded applications ???
| |