On Fri, Mar 20, 2015 at 12:21:34PM -0400, Rik van Riel wrote: > On 03/19/2015 09:43 AM, Matthew Wilcox wrote: > > > 1. Construct struct pages for persistent memory > > 1a. Permanently > > 1b. While the pages are under I/O > > Michael Tsirkin and I have been doing some thinking about what > it would take to allocate struct pages per 2MB area permanently, > and allocate additional struct pages for 4kB pages on demand, > when a 2MB area is broken up into 4kB pages. Ah! I've looked at that a couple of times as well. I asked our database performance team what impact freeing up the memmap would have on their performance. They told me that doubling the amount of memory generally resulted in approximately a 40% performance improvement. So freeing up 1.5% additional memory would result in about 0.6% performance improvement, which I thought was probably too small a return on investment to justify turning memmap into a two-level data structure. Persistent memory might change that calculation somewhat ... but I'm not convinced. Certainly, if we already had the ability to allocate 'struct superpage', I wouldn't be pushing for page-less I/Os, I'd just allocate these data structures for PM. Even if they were 128 bytes in size, that's only a 25MB overhead per 400GB NV-DIMM, which feels quite reasonable to me. > This should work for both DRAM and persistent memory. > > I am still not convinced it is worthwhile to have struct pages > for persistent memory though, but I am willing to change my mind. There's a lot of code out there that relies on struct page being PAGE_SIZE bytes. I'm cool with replacing 'struct page' with 'struct superpage' [1] in the biovec and auditing all of the code which touches it ... but that's going to be a lot of code! I'm not sure it's less code than going directly to 'just do I/O on PFNs'. [1] Please, somebody come up with a better name! -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
