On Tue, Mar 31, 2015 at 10:11:29PM +0000, Elliott, Robert (Server Storage) wrote: > I used fio to test 4 KiB random read and write IOPS > on a 2-socket x86 DDR4 system. With various cache attributes: > > attr read write notes > ---- ---- ----- ----- > UC 37 K 21 K ioremap_nocache > WB 3.6 M 2.5 M ioremap > WC 764 K 3.7 M ioremap_wc > WT <not tested yet> ioremap_wt > > So, although UC and WT are the only modes certain to be safe, > the V1 default of UC provides abysmal performance - worse than > a consumer-class SATA SSD. It doesn't look quite as bad on my setup, but performance is fairly bad here as well. > A solution for x86 is to use the MOVNTI instruction in WB > mode. This non-temporal hint uses a buffer like the write > combining buffer, not filling the cache and not stopping > everything in the CPU. The kernel function __copy_from_user() > uses that instruction (with SFENCE at the end) - see > arch/x86/lib/copy_user_nocache_64.S. > > If I made the change from memcpy() to __copy_from_user() > correctly, that results in: > > attr read write notes > ---- ---- ----- ----- > WB w/NTI 2.4 M 2.6 M __copy_from_user() > WC w/NTI 3.2 M 2.1 M __copy_from_user() That looks a lot better. It doesn't help us with a pmem device mapped directly into userspace using mmap with the DAX infrastructure, though. Note when we want to move to non-temporal copies we'll need to add a new prototype, as __copy_from_user isn't guaranteed to use these, and it is defined to only work on user addresses. That doesn't matter on x86 but would blow up on say sparc or s390. -- 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
