linux-kernel - Re: [PATCH 0/3] remove rw_page() from brd, pmem and btt

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Date:   Mon, 31 Jul 2017 07:16:59 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-kernel@...r.kernel.org, "karam . lee" <karam.lee@....com>,
        Jerome Marchand <jmarchan@...hat.com>,
        Nitin Gupta <ngupta@...are.org>, seungho1.park@....com,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Jens Axboe <axboe@...nel.dk>,
        Vishal Verma <vishal.l.verma@...el.com>,
        linux-nvdimm@...ts.01.org
Subject: Re: [PATCH 0/3] remove rw_page() from brd, pmem and btt

Hi Andrew,

On Fri, Jul 28, 2017 at 02:21:23PM -0700, Andrew Morton wrote:
> On Fri, 28 Jul 2017 10:31:43 -0700 Matthew Wilcox <willy@...radead.org> wrote:
> 
> > On Fri, Jul 28, 2017 at 10:56:01AM -0600, Ross Zwisler wrote:
> > > Dan Williams and Christoph Hellwig have recently expressed doubt about
> > > whether the rw_page() interface made sense for synchronous memory drivers
> > > [1][2].  It's unclear whether this interface has any performance benefit
> > > for these drivers, but as we continue to fix bugs it is clear that it does
> > > have a maintenance burden.  This series removes the rw_page()
> > > implementations in brd, pmem and btt to relieve this burden.
> > 
> > Why don't you measure whether it has performance benefits?  I don't
> > understand why zram would see performance benefits and not other drivers.
> > If it's going to be removed, then the whole interface should be removed,
> > not just have the implementations removed from some drivers.
> 
> Yes please.  Minchan, could you please take a look sometime?

rw_page's gain is reducing of dynamic allocation in swap path
as well as performance gain thorugh avoiding bio allocation.
And it would be important in memory pressure situation.

I guess it comes from bio_alloc mempool. Usually, zram-swap works
in high memory pressure so mempool would be exahusted easily.
It means that mempool wait and repeated alloc would consume the
overhead.

Actually, at that time although Karam reported the gain is 2.4%,
I got a report from production team that the gain in corner case
(e.g., animation playing is smooth) would be much higher than
expected.