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SubjectRE: another pmem variant V2
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> -----Original Message-----
> From: Christoph Hellwig [mailto:hch@lst.de]
> Sent: Wednesday, April 1, 2015 2:26 AM
> To: Elliott, Robert (Server Storage)
> Cc: Christoph Hellwig; linux-nvdimm@ml01.01.org; linux-
> fsdevel@vger.kernel.org; linux-kernel@vger.kernel.org; x86@kernel.org;
> ross.zwisler@linux.intel.com; axboe@kernel.dk; boaz@plexistor.com; 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.



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