RE: another pmem variant V2

[Elliott, Robert (Server Storage)]

> -----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.

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