On Wed, Oct 11, 2006 at 11:49:10AM -0500, Steve Lord wrote: > David Chinner wrote: > >On Tue, Oct 10, 2006 at 10:19:04AM +0100, Christoph Hellwig wrote: > >>On Mon, Oct 09, 2006 at 09:15:28PM -0500, Steve Lord wrote: > >>>Hi Dave, > >>> > >>>My recollection is that it used to default to on, it was disabled > >>>because it needs to map the buffer into a single contiguous chunk > >>>of kernel memory. This was placing a lot of pressure on the memory > >>>remapping code, so we made it not default to on as reworking the > >>>code to deal with non contig memory was looking like a major > >>>effort. > >>Exactly. The code works but tends to go OOM pretty fast at least > >>when the dir blocksize code is bigger than the page size. I should > >>give the code a spin on my ppc box with 64k pages if it works better > >>there. > > > >The pagebuf code doesn't use high-order allocations anymore; it uses > >scatter lists and remapping to allow physically discontiguous pages > >in a multi-page buffer. That is, the pages are sourced via > >find_or_create_page() from the address space of the backing device, > >and then mapped via vmap() to provide a virtually contigous mapping > >of the multi-page buffer. > > > >So I don't think this problem exists anymore... > > I was not referring to high order allocations here, but the overhead > of doing address space remapping every time a directory is accessed. Ah - ok. contig -> non-contig and OOM is usually discussed in the context of higher order allocations failing. FWIW, I've not noticed any extra overhead - the CPU usage seems to grow roughly linearly with the increase in directory operations done as a result of higher throughput for the same number of I/Os. I'll have a look at the Vm stats, though, next time I run a comparison to see how bad this is. Thanks for the clarification, Steve. Cheers, Dave. -- Dave Chinner Principal Engineer SGI Australian Software Group - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
