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

Minchan Kim
2017-08-04T08:17:49Z

On Fri, Aug 04, 2017 at 12:54:41PM +0900, Minchan Kim wrote:
> On Thu, Aug 03, 2017 at 03:13:35PM -0600, Ross Zwisler wrote:
> > On Thu, Aug 03, 2017 at 09:13:15AM +0900, Minchan Kim wrote:
> > > Hi Ross,
> > >
> > > On Wed, Aug 02, 2017 at 04:13:59PM -0600, Ross Zwisler wrote:
> > > > On Fri, Jul 28, 2017 at 10:31:43AM -0700, Matthew Wilcox 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.
> > > >
> > > > Okay, I've run a bunch of performance tests with the PMEM and with BTT entry
> > > > points for rw_pages() in a swap workload, and in all cases I do see an
> > > > improvement over the code when rw_pages() is removed. Here are the results
> > > > from my random lab box:
> > > >
> > > > Average latency of swap_writepage()
> > > > +------+------------+---------+-------------+
> > > > | | no rw_page | rw_page | Improvement |
> > > > +-------------------------------------------+
> > > > | PMEM | 5.0 us | 4.7 us | 6% |
> > > > +-------------------------------------------+
> > > > | BTT | 6.8 us | 6.1 us | 10% |
> > > > +------+------------+---------+-------------+
> > > >
> > > > Average latency of swap_readpage()
> > > > +------+------------+---------+-------------+
> > > > | | no rw_page | rw_page | Improvement |
> > > > +-------------------------------------------+
> > > > | PMEM | 3.3 us | 2.9 us | 12% |
> > > > +-------------------------------------------+
> > > > | BTT | 3.7 us | 3.4 us | 8% |
> > > > +------+------------+---------+-------------+
> > > >
> > > > The workload was pmbench, a memory benchmark, run on a system where I had
> > > > severely restricted the amount of memory in the system with the 'mem' kernel
> > > > command line parameter. The benchmark was set up to test more memory than I
> > > > allowed the OS to have so it spilled over into swap.
> > > >
> > > > The PMEM or BTT device was set up as my swap device, and during the test I got
> > > > a few hundred thousand samples of each of swap_writepage() and
> > > > swap_writepage(). The PMEM/BTT device was just memory reserved with the
> > > > memmap kernel command line parameter.
> > > >
> > > > Thanks, Matthew, for asking for performance data. It looks like removing this
> > > > code would have been a mistake.
> > >
> > > By suggestion of Christoph Hellwig, I made a quick patch which does IO without
> > > dynamic bio allocation for swap IO. Actually, it's not formal patch to be
> > > worth to send mainline yet but I believe it's enough to test the improvement.
> > >
> > > Could you test patchset on pmem and btt without rw_page?
> > >
> > > For working the patch, block drivers need to declare it's synchronous IO
> > > device via BDI_CAP_SYNC but if it's hard, you can just make every swap IO
> > > comes from (sis->flags & SWP_SYNC_IO) with removing condition check
> > >
> > > if (!(sis->flags & SWP_SYNC_IO)) in swap_[read|write]page.
> > >
> > > Patchset is based on 4.13-rc3.
> >
> > Thanks for the patch, here are the updated results from my test box:
> >
> > Average latency of swap_writepage()
> > +------+------------+---------+---------+
> > | | no rw_page | minchan | rw_page |
> > +----------------------------------------
> > | PMEM | 5.0 us | 4.98 us | 4.7 us |
> > +----------------------------------------
> > | BTT | 6.8 us | 6.3 us | 6.1 us |
> > +------+------------+---------+---------+
> >
> > Average latency of swap_readpage()
> > +------+------------+---------+---------+
> > | | no rw_page | minchan | rw_page |
> > +----------------------------------------
> > | PMEM | 3.3 us | 3.27 us | 2.9 us |
> > +----------------------------------------
> > | BTT | 3.7 us | 3.44 us | 3.4 us |
> > +------+------------+---------+---------+
> >
> > I've added another digit in precision in some cases to help differentiate the
> > various results.
> >
> > In all cases your patches did perform better than with the regularly allocated
> > BIO, but again for all cases the rw_page() path was the fastest, even if only
> > marginally.
>
> Thanks for the testing. Your testing number is within noise level?
>
> I cannot understand why PMEM doesn't have enough gain while BTT is significant
> win(8%). I guess no rw_page with BTT testing had more chances to wait bio dynamic
> allocation and mine and rw_page testing reduced it significantly. However,
> in no rw_page with pmem, there wasn't many cases to wait bio allocations due
> to the device is so fast so the number comes from purely the number of
> instructions has done. At a quick glance of bio init/submit, it's not trivial
> so indeed, i understand where the 12% enhancement comes from but I'm not sure
> it's really big difference in real practice at the cost of maintaince burden.

I tested pmbench 10 times in my local machine(4 core) with zram-swap.
In my machine, even, on-stack bio is faster than rw_page. Unbelievable.

I guess it's really hard to get stable result in severe memory pressure.
It would be a result within noise level(see below stddev).
So, I think it's hard to conclude rw_page is far faster than onstack-bio.

rw_page
avg 5.54us
stddev 8.89%
max 6.02us
min 4.20us

onstack bio
avg 5.27us
stddev 13.03%
max 5.96us
min 3.55us