> -----Original Message----- > From: Christoph Hellwig [mailto:hch@xxxxxx] > Sent: Wednesday, April 1, 2015 2:26 AM > To: Elliott, Robert (Server Storage) > Cc: Christoph Hellwig; linux-nvdimm@xxxxxxxxxxx; linux- > fsdevel@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; x86@xxxxxxxxxx; > ross.zwisler@xxxxxxxxxxxxxxx; axboe@xxxxxxxxx; boaz@xxxxxxxxxxxxx; Kani, > Toshimitsu > Subject: Re: another pmem variant V2 > > 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. Here are some updated numbers including: * WT (writethrough) cache attribute * memcpy that uses non-temporal stores (MOVNTDQ) to the persistent memory for block writes (rather than MOVNTI) * memcpy that uses non-temporal loads (MOVNTDQA) from the persistent memory for block reads Attr Copy Read IOPS Write IOPS ==== ==== ========= ========== UC memcpy 36 K 22 K UC NT rd,wr 513 K 326 K WB memcpy 3.4 M 2.5 M WB NT rd,wr 3.3 M 3.5 M WC memcpy 776 K 3.5 M WC NT rd,wr 3.0 M 3.9 M WT memcpy 2.1 M 22 K WT NT rd,wr 3.3 M 2.1 M a few other variations yielded the peak numbers: WC NT rd only 3.2 M 4.1 M WC NT wr only 712 K 4.6 M WT NT wr only 2.6 M 4.0 M There are lots of tuning considerations for those memcpy functions - how far to unroll the loop, whether to include PRFETCHNTA instructions, etc. -- 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
