Re: [PATCH v2 0/6] RFC: gup+dma: tracking dma-pinned pages

Tom Talpey
2018-11-28T01:29:13Z

On 11/21/2018 5:06 PM, John Hubbard wrote:
> On 11/21/18 8:49 AM, Tom Talpey wrote:
>> On 11/21/2018 1:09 AM, John Hubbard wrote:
>>> On 11/19/18 10:57 AM, Tom Talpey wrote:
>>>> ~14000 4KB read IOPS is really, really low for an NVMe disk.
>>>
>>> Yes, but Jan Kara's original config file for fio is *intended* to highlight
>>> the get_user_pages/put_user_pages changes. It was *not* intended to get max
>>> performance,  as you can see by the numjobs and direct IO parameters:
>>>
>>> cat fio.conf
>>> [reader]
>>> direct=1
>>> ioengine=libaio
>>> blocksize=4096
>>> size=1g
>>> numjobs=1
>>> rw=read
>>> iodepth=64
>>
>> To be clear - I used those identical parameters, on my lower-spec
>> machine, and got 400,000 4KB read IOPS. Those results are nearly 30x
>> higher than yours!
>
> OK, then something really is wrong here...
>
>>
>>> So I'm thinking that this is not a "tainted" test, but rather, we're constraining
>>> things a lot with these choices. It's hard to find a good test config to run that
>>> allows decisions, but so far, I'm not really seeing anything that says "this
>>> is so bad that we can't afford to fix the brokenness." I think.
>>
>> I'm not suggesting we tune the benchmark, I'm suggesting the results
>> on your system are not meaningful since they are orders of magnitude
>> low. And without meaningful data it's impossible to see the performance
>> impact of the change...
>>
>>>> Can you confirm what type of hardware you're running this test on?
>>>> CPU, memory speed and capacity, and NVMe device especially?
>>>>
>>>> Tom.
>>>
>>> Yes, it's a nice new system, I don't expect any strange perf problems:
>>>
>>> CPU: Intel(R) Core(TM) i7-7800X CPU @ 3.50GHz
>>>      (Intel X299 chipset)
>>> Block device: nvme-Samsung_SSD_970_EVO_250GB
>>> DRAM: 32 GB
>>
>> The Samsung Evo 970 250GB is speced to yield 200,000 random read IOPS
>> with a 4KB QD32 workload:
>>
>>
>> https://www.samsung.com/us/computing/memory-storage/solid-state-drives/ssd-970-evo-nvme-m-2-250gb-mz-v7e250bw/#specs
>>
>> And the I7-7800X is a 6-core processor (12 hyperthreads).
>>
>>> So, here's a comparison using 20 threads, direct IO, for the baseline vs.
>>> patched kernel (below). Highlights:
>>>
>>>     -- IOPS are similar, around 60k.
>>>     -- BW gets worse, dropping from 290 to 220 MB/s.
>>>     -- CPU is well under 100%.
>>>     -- latency is incredibly long, but...20 threads.
>>>
>>> Baseline:
>>>
>>> $ ./run.sh
>>> fio configuration:
>>> [reader]
>>> ioengine=libaio
>>> blocksize=4096
>>> size=1g
>>> rw=read
>>> group_reporting
>>> iodepth=256
>>> direct=1
>>> numjobs=20
>>
>> Ouch - 20 threads issuing 256 io's each!? Of course latency skyrockets.
>> That's going to cause tremendous queuing, and context switching, far
>> outside of the get_user_pages() change.
>>
>> But even so, it only brings IOPS to 74.2K, which is still far short of
>> the device's 200K spec.
>>
>> Comparing anyway:
>>
>>
>>> Patched:
>>>
>>> -------- Running fio:
>>> reader: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=libaio, iodepth=256
>>> ...
>>> fio-3.3
>>> Starting 20 processes
>>> Jobs: 13 (f=8): [_(1),R(1),_(1),f(1),R(2),_(1),f(2),_(1),R(1),f(1),R(1),f(1),R(1),_(2),R(1),_(1),R(1)][97.9%][r=229MiB/s,w=0KiB/s][r=58.5k,w=0 IOPS][eta 00m:02s]
>>> reader: (groupid=0, jobs=20): err= 0: pid=2104: Tue Nov 20 22:01:58 2018
>>>     read: IOPS=56.8k, BW=222MiB/s (232MB/s)(20.0GiB/92385msec)
>>> ...
>>> Thoughts?
>>
>> Concern - the 74.2K IOPS unpatched drops to 56.8K patched!
>
> ACK. :)
>
>>
>> What I'd really like to see is to go back to the original fio parameters
>> (1 thread, 64 iodepth) and try to get a result that gets at least close
>> to the speced 200K IOPS of the NVMe device. There seems to be something
>> wrong with yours, currently.
>
> I'll dig into what has gone wrong with the test. I see fio putting data files
> in the right place, so the obvious "using the wrong drive" is (probably)
> not it. Even though it really feels like that sort of thing. We'll see.
>
>>
>> Then of course, the result with the patched get_user_pages, and
>> compare whichever of IOPS or CPU% changes, and how much.
>>
>> If these are within a few percent, I agree it's good to go. If it's
>> roughly 25% like the result just above, that's a rocky road.
>>
>> I can try this after the holiday on some basic hardware and might
>> be able to scrounge up better. Can you post that github link?
>>
>
> Here:
>
> git@github.com:johnhubbard/linux (branch: gup_dma_testing)

I'm super-limited here this week hardware-wise and have not been able
to try testing with the patched kernel.

I was able to compare my earlier quick test with a Bionic 4.15 kernel
(400K IOPS) against a similar 4.20rc3 kernel, and the rate dropped to
~_375K_ IOPS. Which I found perhaps troubling. But it was only a quick
test, and without your change.

Say, that branch reports it has not had a commit since June 30. Is that
the right one? What about gup_dma_for_lpc_2018?

Tom.