Re: another pmem variant V2

[Christoph Hellwig <hch@lst.de>]

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.