Re: [PATCH 4/6] mm: introduce page->dma_pinned_flags, _count

[Dave Chinner]

On Tue, Nov 06, 2018 at 12:00:06PM +0100, Jan Kara wrote:
> On Tue 06-11-18 13:47:15, Dave Chinner wrote:
> > On Mon, Nov 05, 2018 at 04:26:04PM -0800, John Hubbard wrote:
> > > On 11/5/18 1:54 AM, Jan Kara wrote:
> > > > Hmm, have you tried larger buffer sizes? Because synchronous 8k IO isn't
> > > > going to max-out NVME iops by far. Can I suggest you install fio [1] (it
> > > > has the advantage that it is pretty much standard for a test like this so
> > > > everyone knows what the test does from a glimpse) and run with it something
> > > > like the following workfile:
> > > > 
> > > > [reader]
> > > > direct=1
> > > > ioengine=libaio
> > > > blocksize=4096
> > > > size=1g
> > > > numjobs=1
> > > > rw=read
> > > > iodepth=64
> > > > 
> > > > And see how the numbers with and without your patches compare?
> > > > 
> > > > 								Honza
> > > > 
> > > > [1] https://github.com/axboe/fio
> > > 
> > > That program is *very* good to have. Whew. Anyway, it looks like read bandwidth 
> > > is approximately 74 MiB/s with my patch (it varies a bit, run to run),
> > > as compared to around 85 without the patch, so still showing about a 20%
> > > performance degradation, assuming I'm reading this correctly.
> > > 
> > > Raw data follows, using the fio options you listed above:
> > > 
> > > Baseline (without my patch):
> > > ---------------------------- 
> > ....
> > >      lat (usec): min=179, max=14003, avg=2913.65, stdev=1241.75
> > >     clat percentiles (usec):
> > >      |  1.00th=[ 2311],  5.00th=[ 2343], 10.00th=[ 2343], 20.00th=[ 2343],
> > >      | 30.00th=[ 2343], 40.00th=[ 2376], 50.00th=[ 2376], 60.00th=[ 2376],
> > >      | 70.00th=[ 2409], 80.00th=[ 2933], 90.00th=[ 4359], 95.00th=[ 5276],
> > >      | 99.00th=[ 8291], 99.50th=[ 9110], 99.90th=[10945], 99.95th=[11469],
> > >      | 99.99th=[12256]
> > .....
> > > Modified (with my patch):
> > > ---------------------------- 
> > .....
> > >      lat (usec): min=81, max=15766, avg=3496.57, stdev=1450.21
> > >     clat percentiles (usec):
> > >      |  1.00th=[ 2835],  5.00th=[ 2835], 10.00th=[ 2835], 20.00th=[ 2868],
> > >      | 30.00th=[ 2868], 40.00th=[ 2868], 50.00th=[ 2868], 60.00th=[ 2900],
> > >      | 70.00th=[ 2933], 80.00th=[ 3425], 90.00th=[ 5080], 95.00th=[ 6259],
> > >      | 99.00th=[10159], 99.50th=[11076], 99.90th=[12649], 99.95th=[13435],
> > >      | 99.99th=[14484]
> > 
> > So it's adding at least 500us of completion latency to every IO?
> > I'd argue that the IO latency impact is far worse than the a 20%
> > throughput drop.
> 
> Hum, right. So for each IO we have to remove the page from LRU on submit

Which cost us less then 10us on average:

	slat (usec): min=13, max=3855, avg=44.17, stdev=61.18
vs
	slat (usec): min=18, max=4378, avg=52.59, stdev=63.66

> and then put it back on IO completion (which is going to race with new
> submits so LRU lock contention might be an issue).

Removal has to take the same LRU lock, so I don't think contention
is the problem here. More likely the overhead is in selecting the
LRU to put it on. e.g. list_lru_from_kmem() which may well be doing
a memcg lookup.

> Spending 500 us on that
> is not unthinkable when the lock is contended but it is more expensive than
> I'd have thought. John, could you perhaps profile where the time is spent?

That'll tell us for sure :)

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx