linux-kernel - Re: another pmem variant V2

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Message-ID: <20150402180329.GA8503@gmail.com>
Date:	Thu, 2 Apr 2015 20:03:30 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Christoph Hellwig <hch@....de>
Cc:	"Elliott, Robert (Server Storage)" <Elliott@...com>,
	"linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	"boaz@...xistor.com" <boaz@...xistor.com>,
	"Kani, Toshimitsu" <toshi.kani@...com>
Subject: Re: another pmem variant V2


* Christoph Hellwig <hch@....de> wrote:

> On Thu, Apr 02, 2015 at 03:11:36PM +0000, Elliott, Robert (Server Storage) wrote:
> > 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.
> 
> Looks like we should aіm for WC + NT would be a good start.
> 
> Can you prepare a patch to add your NT memcpy variants and
> a second one to use them in the pmem driver?

So we already have NT memcpy variants, see the copy_*_user_*_nocache() 
primitives in arch/x86/. They could be used almost straight away for 
kernel memory as well, as kernel buffers will not fault.

Thanks,

	Ingo
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