On 4/13/21 5:14 PM, Dave Chinner wrote:
> On Tue, Apr 13, 2021 at 10:13:24AM -0600, Jens Axboe wrote:
>> On 4/13/21 1:51 AM, SeongJae Park wrote:
>>> From: SeongJae Park <sjpark@xxxxxxxxx>
>>>
>>> Hello,
>>>
>>>
>>> Very interesting work, thank you for sharing this :)
>>>
>>> On Tue, 13 Apr 2021 00:56:17 -0600 Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>>>
>>>> What's new in v2
>>>> ================
>>>> Special thanks to Jens Axboe for reporting a regression in buffered
>>>> I/O and helping test the fix.
>>>
>>> Is the discussion open? If so, could you please give me a link?
>>
>> I wasn't on the initial post (or any of the lists it was posted to), but
>> it's on the google page reclaim list. Not sure if that is public or not.
>>
>> tldr is that I was pretty excited about this work, as buffered IO tends
>> to suck (a lot) for high throughput applications. My test case was
>> pretty simple:
>>
>> Randomly read a fast device, using 4k buffered IO, and watch what
>> happens when the page cache gets filled up. For this particular test,
>> we'll initially be doing 2.1GB/sec of IO, and then drop to 1.5-1.6GB/sec
>> with kswapd using a lot of CPU trying to keep up. That's mainline
>> behavior.
>
> I see this exact same behaviour here, too, but I RCA'd it to
> contention between the inode and memory reclaim for the mapping
> structure that indexes the page cache. Basically the mapping tree
> lock is the contention point here - you can either be adding pages
> to the mapping during IO, or memory reclaim can be removing pages
> from the mapping, but we can't do both at once.
>
> So we end up with kswapd spinning on the mapping tree lock like so
> when doing 1.6GB/s in 4kB buffered IO:
>
> - 20.06% 0.00% [kernel] [k] kswapd ▒
> - 20.06% kswapd ▒
> - 20.05% balance_pgdat ▒
> - 20.03% shrink_node ▒
> - 19.92% shrink_lruvec ▒
> - 19.91% shrink_inactive_list ▒
> - 19.22% shrink_page_list ▒
> - 17.51% __remove_mapping ▒
> - 14.16% _raw_spin_lock_irqsave ▒
> - 14.14% do_raw_spin_lock ▒
> __pv_queued_spin_lock_slowpath ▒
> - 1.56% __delete_from_page_cache ▒
> 0.63% xas_store ▒
> - 0.78% _raw_spin_unlock_irqrestore ▒
> - 0.69% do_raw_spin_unlock ▒
> __raw_callee_save___pv_queued_spin_unlock ▒
> - 0.82% free_unref_page_list ▒
> - 0.72% free_unref_page_commit ▒
> 0.57% free_pcppages_bulk ▒
>
> And these are the processes consuming CPU:
>
> 5171 root 20 0 1442496 5696 1284 R 99.7 0.0 1:07.78 fio
> 1150 root 20 0 0 0 0 S 47.4 0.0 0:22.70 kswapd1
> 1146 root 20 0 0 0 0 S 44.0 0.0 0:21.85 kswapd0
> 1152 root 20 0 0 0 0 S 39.7 0.0 0:18.28 kswapd3
> 1151 root 20 0 0 0 0 S 15.2 0.0 0:12.14 kswapd2
Here's my profile when memory reclaim is active for the above mentioned
test case. This is a single node system, so just kswapd. It's using around
40-45% CPU:
43.69% kswapd0 [kernel.vmlinux] [k] xas_create
|
---ret_from_fork
kthread
kswapd
balance_pgdat
shrink_node
shrink_lruvec
shrink_inactive_list
shrink_page_list
__delete_from_page_cache
xas_store
xas_create
16.88% kswapd0 [kernel.vmlinux] [k] queued_spin_lock_slowpath
|
---ret_from_fork
kthread
kswapd
balance_pgdat
shrink_node
shrink_lruvec
|
--16.82%--shrink_inactive_list
|
--16.55%--shrink_page_list
|
--16.26%--_raw_spin_lock_irqsave
queued_spin_lock_slowpath
9.89% kswapd0 [kernel.vmlinux] [k] shrink_page_list
|
---ret_from_fork
kthread
kswapd
balance_pgdat
shrink_node
shrink_lruvec
shrink_inactive_list
shrink_page_list
5.46% kswapd0 [kernel.vmlinux] [k] xas_init_marks
|
---ret_from_fork
kthread
kswapd
balance_pgdat
shrink_node
shrink_lruvec
shrink_inactive_list
shrink_page_list
|
--5.41%--__delete_from_page_cache
xas_init_marks
4.42% kswapd0 [kernel.vmlinux] [k] __delete_from_page_cache
|
---ret_from_fork
kthread
kswapd
balance_pgdat
shrink_node
shrink_lruvec
shrink_inactive_list
|
--4.40%--shrink_page_list
__delete_from_page_cache
2.82% kswapd0 [kernel.vmlinux] [k] isolate_lru_pages
|
---ret_from_fork
kthread
kswapd
balance_pgdat
shrink_node
shrink_lruvec
|
|--1.43%--shrink_active_list
| isolate_lru_pages
|
--1.39%--shrink_inactive_list
isolate_lru_pages
1.99% kswapd0 [kernel.vmlinux] [k] free_pcppages_bulk
|
---ret_from_fork
kthread
kswapd
balance_pgdat
shrink_node
shrink_lruvec
shrink_inactive_list
shrink_page_list
free_unref_page_list
free_unref_page_commit
free_pcppages_bulk
1.79% kswapd0 [kernel.vmlinux] [k] _raw_spin_lock_irqsave
|
---ret_from_fork
kthread
kswapd
balance_pgdat
|
--1.76%--shrink_node
shrink_lruvec
shrink_inactive_list
|
--1.72%--shrink_page_list
_raw_spin_lock_irqsave
1.02% kswapd0 [kernel.vmlinux] [k] workingset_eviction
|
---ret_from_fork
kthread
kswapd
balance_pgdat
shrink_node
shrink_lruvec
shrink_inactive_list
|
--1.00%--shrink_page_list
workingset_eviction
> i.e. when memory reclaim kicks in, the read process has 20% less
> time with exclusive access to the mapping tree to insert new pages.
> Hence buffered read performance goes down quite substantially when
> memory reclaim kicks in, and this really has nothing to do with the
> memory reclaim LRU scanning algorithm.
>
> I can actually get this machine to pin those 5 processes to 100% CPU
> under certain conditions. Each process is spinning all that extra
> time on the mapping tree lock, and performance degrades further.
> Changing the LRU reclaim algorithm won't fix this - the workload is
> solidly bound by the exclusive nature of the mapping tree lock and
> the number of tasks trying to obtain it exclusively...
I've seen way worse than the above as well, it's just my go-to easy test
case for "man I wish buffered IO didn't suck so much".
>> The initial posting of this patchset did no better, in fact it did a bit
>> worse. Performance dropped to the same levels and kswapd was using as
>> much CPU as before, but on top of that we also got excessive swapping.
>> Not at a high rate, but 5-10MB/sec continually.
>>
>> I had some back and forths with Yu Zhao and tested a few new revisions,
>> and the current series does much better in this regard. Performance
>> still dips a bit when page cache fills, but not nearly as much, and
>> kswapd is using less CPU than before.
>
> Profiles would be interesting, because it sounds to me like reclaim
> *might* be batching page cache removal better (e.g. fewer, larger
> batches) and so spending less time contending on the mapping tree
> lock...
>
> IOWs, I suspect this result might actually be a result of less lock
> contention due to a change in batch processing characteristics of
> the new algorithm rather than it being a "better" algorithm...
See above - let me know if you want to see more specific profiling as
well.
--
Jens Axboe
