On 12/11/19 6:29 PM, Jens Axboe wrote:
> On 12/11/19 6:22 PM, Linus Torvalds wrote:
>> On Wed, Dec 11, 2019 at 5:11 PM Jens Axboe <axboe@xxxxxxxxx> wrote:
>>>
>>> 15K is likely too slow to really show an issue, I'm afraid. The 970
>>> is no slouch, but your crypt setup will likely hamper it a lot. You
>>> don't have a non-encrypted partition on it?
>>
>> No. I normally don't need all that much disk, so I've never upgraded
>> my ssd from the 512G size.
>>
>> Which means that it's actually half full or so, and I never felt like
>> "I should keep an unencrypted partition for IO testing", since I don't
>> generally _do_ any IO testing.
>>
>> I can get my load up with "numjobs=8" and get my iops up to the 100k
>> range, though.
>>
>> But kswapd doesn't much seem to care, the CPU percentage actually does
>> _down_ to 0.39% when I try that. Probably simply because now my CPU's
>> are busy, so they are running at 4.7Ghz instead of the 800Mhz "mostly
>> idle" state ...
>>
>> I guess I should be happy. It does mean that the situation you see
>> isn't exactly the normal case. I understand why you want to do the
>> non-cached case, but the case I think it the worrisome one is the
>> regular buffered one, so that's what I'm testing (not even trying the
>> noaccess patches).
>>
>> So from your report I went "uhhuh, that sounds like a bug". And it
>> appears that it largely isn't - you're seeing it because of pushing
>> the IO subsystem by another order of magnitude (and then I agree that
>> "under those kinds of IO loads, caching just won't help")
>
> I'd very much argue that it IS a bug, maybe just doesn't show on your
> system. My test box is a pretty standard 2 socket system, 24 cores / 48
> threads, 2 nodes. The last numbers I sent were 100K IOPS, so nothing
> crazy, and granted that's only 10% kswapd cpu time, but that still seems
> very high for those kinds of rates. I'm surprised you see essentially no
> kswapd time for the same data rate.
>
> We'll keep poking here, I know Johannes is spending some time looking
> into the reclaim side.
Out of curiosity, just tried it on my laptop, which also has some
samsung drive. Using 8 jobs, I get around 100K IOPS too, and this
is my top listing:
23308 axboe 20 0 623156 1304 8 D 10.3 0.0 0:03.81 fio
23309 axboe 20 0 623160 1304 8 D 10.3 0.0 0:03.81 fio
23311 axboe 20 0 623168 1304 8 D 10.3 0.0 0:03.82 fio
23313 axboe 20 0 623176 1304 8 D 10.3 0.0 0:03.82 fio
23314 axboe 20 0 623180 1304 8 D 10.3 0.0 0:03.81 fio
162 root 20 0 0 0 0 S 9.9 0.0 0:12.97 kswapd0
23307 axboe 20 0 623152 1304 8 D 9.9 0.0 0:03.84 fio
23310 axboe 20 0 623164 1304 8 D 9.9 0.0 0:03.81 fio
23312 axboe 20 0 623172 1304 8 D 9.9 0.0 0:03.80 fio
kswapd is between 9-11% the whole time, and the profile looks very
similar to what I saw on my test box:
35.79% kswapd0 [kernel.vmlinux] [k] xas_create
9.97% kswapd0 [kernel.vmlinux] [k] free_pcppages_bulk
9.94% kswapd0 [kernel.vmlinux] [k] isolate_lru_pages
7.78% kswapd0 [kernel.vmlinux] [k] shrink_page_list
3.78% kswapd0 [kernel.vmlinux] [k] xas_clear_mark
3.08% kswapd0 [kernel.vmlinux] [k] workingset_eviction
2.48% kswapd0 [kernel.vmlinux] [k] __isolate_lru_page
2.06% kswapd0 [kernel.vmlinux] [k] page_mapping
1.95% kswapd0 [kernel.vmlinux] [k] __remove_mapping
So now I'm even more puzzled why your (desktop?) doesn't show it, it
must be more potent than my x1 laptop. But for me, the laptop and 2
socket test box show EXACTLY the same behavior, laptop is just too slow
to make it really pathological.
--
Jens Axboe
