On 03/20/2015 04:31 PM, Matthew Wilcox wrote: > 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. Agreed, it should not be done for memory savings alone, but only if it helps improve all kinds of other things. >> 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'. Totally agreed here. I see absolutely no advantage to teaching the IO layer about a "struct superpage" when it could operate on PFNs just as easily. -- All rights reversed -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
