Re: [PATCH v1 2/6] fs: use on-stack-bio if backing device has BDI_CAP_SYNC capability

[Minchan Kim <minchan@kernel.org>]

Hi Jens,

On Wed, Aug 16, 2017 at 09:56:12AM -0600, Jens Axboe wrote:
> On 08/15/2017 10:48 PM, Minchan Kim wrote:
> > Hi Jens,
> > 
> > On Mon, Aug 14, 2017 at 10:17:09AM -0600, Jens Axboe wrote:
> >> On 08/14/2017 09:38 AM, Jens Axboe wrote:
> >>> On 08/14/2017 09:31 AM, Minchan Kim wrote:
> >>>>> Secondly, generally you don't have slow devices and fast devices
> >>>>> intermingled when running workloads. That's the rare case.
> >>>>
> >>>> Not true. zRam is really popular swap for embedded devices where
> >>>> one of low cost product has a really poor slow nand compared to
> >>>> lz4/lzo [de]comression.
> >>>
> >>> I guess that's true for some cases. But as I said earlier, the recycling
> >>> really doesn't care about this at all. They can happily coexist, and not
> >>> step on each others toes.
> >>
> >> Dusted it off, result is here against -rc5:
> >>
> >> http://git.kernel.dk/cgit/linux-block/log/?h=cpu-alloc-cache
> >>
> >> I'd like to split the amount of units we cache and the amount of units
> >> we free, right now they are both CPU_ALLOC_CACHE_SIZE. This means that
> >> once we hit that count, we free all of the, and then store the one we
> >> were asked to free. That always keeps 1 local, but maybe it'd make more
> >> sense to cache just free CPU_ALLOC_CACHE_SIZE/2 (or something like that)
> >> so that we retain more than 1 per cpu in case and app preempts when
> >> sleeping for IO and the new task on that CPU then issues IO as well.
> >> Probably minor.
> >>
> >> Ran a quick test on nullb0 with 32 sync readers. The test was O_DIRECT
> >> on the block device, so I disabled the __blkdev_direct_IO_simple()
> >> bypass. With the above branch, we get ~18.0M IOPS, and without we get
> >> ~14M IOPS. Both ran with iostats disabled, to avoid any interference
> >> from that.
> > 
> > Looks promising.
> > If recycling bio works well enough, I think we don't need to introduce
> > new split in the path for on-stack bio.
> > I will test your version on zram-swap!
> 
> Thanks, let me know how it goes. It's quite possible that we'll need
> a few further tweaks, but at least the basis should be there.

Sorry for my late reply.

I just finished the swap-in testing in with zram-swap which is critical
for the latency.

For the testing, I made a memcc and put $NR_CPU(mine is 12) processes
in there and each processes consumes 1G so total is 12G while my system
has 16GB memory so there was no global reclaim.
Then, echo 1 > /mnt/memcg/group/force.empty to swap all pages out and
then the programs wait my signal to swap in and I trigger the signal
to every processes to swap in every pages and measures elapsed time
for the swapin.

the value is average usec time elapsed swap-in 1G pages for each process
and I repeated it 10times and stddev is very stable.

swapin:
base(with rw_page)      1100806.73(100.00%)
no-rw_page              1146856.95(104.18%)
Jens's pcp              1146910.00(104.19%)
onstack-bio             1114872.18(101.28%)

In my test, there is no difference between dynamic bio allocation
(i.e., no-rwpage) and pcp approch but onstack-bio is much faster
so it's almost same with rw_page.

swapout test is to measure elapsed time for "echo 1 > /mnt/memcg/test_group/force.empty'
so it's sec unit.

swapout:
base(with rw_page)      7.72(100.00%)
no-rw_page              8.36(108.29%)
Jens's pcp              8.31(107.64%)
onstack-bio             8.19(106.09%)

rw_page's swapout is 6% or more than faster than else.

I tried pmbenchmak with no memcg to see the performance in global reclaim.
Also, I executed background IO job which reads data from HDD.
The value is average usec time elapsed for a page access so smaller is
better.

base(with rw_page)              14.42(100.00%)
no-rw_page                      15.66(108.60%)
Jens's pcp                      15.81(109.64%)
onstack-bio                     15.42(106.93%)

It's similar to swapout test in memcg.
6% or more is not trivial so I doubt we can remove rw_page
at this moment. :(

I will look into the detail with perf.
If you have further optimizations or suggestions, Feel free to
say that. I am happy to test it.

Thanks.
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