Re: [RFC] mm: stress-ng --mremap triggers severe lruvec lock contention in populate/unmap paths

Joseph Salisbury
2026-04-09T17:26:40Z

On 4/9/26 12:37 PM, Haakon Bugge wrote:
>> On 8 Apr 2026, at 16:27, Joseph Salisbury <joseph.salisbury@oracle.com> wrote:
>>
>>
>>
>> On 4/8/26 4:09 AM, David Hildenbrand (Arm) wrote:
>>>>> It was also found that adding '--mremap-numa' changes the behavior
>>>>> substantially:
>>>> "assign memory mapped pages to randomly selected NUMA nodes. This is
>>>> disabled for systems that do not support NUMA."
>>>>
>>>> so this is just sharding your lock contention across your NUMA nodes (you
>>>> have an lruvec per node).
>>>>
>>>>> stress-ng --mremap 8192 --mremap-bytes 4K --timeout 30 --mremap-numa
>>>>> --metrics-brief
>>>>>
>>>>> mremap 2570798 29.39 8.06 106.23 87466.50 22494.74
>>>>>
>>>>> So it's possible that either actual swapping, or the mbind(...,
>>>>> MPOL_MF_MOVE) path used by '--mremap-numa', removes most of the excessive
>>>>> system time.
>>>>>
>>>>> Does this look like a known MM scalability issue around short-lived
>>>>> MAP_POPULATE / munmap churn?
>>>> Yes. Is this an actual issue on some workload?
>>> Same thought, it's unclear to me why we should care here. In particular,
>>> when talking about excessive use of zero-filled pages.
>>>
>> Currently this is only showing up with that particular stress test. We will try John's patch and provide feedback.
>>
>> Thanks for all the feedback, everyone!
> I reported this internally and have worked with Joseph on it. I tested v7.0-rc7-68-g7f87a5ea75f01 ("-"), "Base", vs. ditto plus John Hubbard's patch ("+"), "Test".
>
> Stress-ng command: stress-ng --mremap 8192 --mremap-bytes 4K --timeout 30 --metrics-brief
>
> System is an AMD EPYC 9J45:
> NUMA node(s): 2
> NUMA node0 CPU(s): 0-127,256-383
> NUMA node1 CPU(s): 128-255,384-511
>
> The stress-ng command was run ten times and here are the averages and pstdev:
>
> bogo ops/s pstdev system time pstdev
> (realtime)
> --------------------------------------------
> - 3192638 35% 24041 32%
> + 3657904 5% 15278 0%
>
> This is 15% improvement in bogo ops/s (realtime) and a decent 36% reduction in system time.
>
> I shamelessly copied and modified the fio command from [1]. I ran:
>
> # fio -filename=/dev/nvme0n1 -direct=0 -thread -size=1024G -rwmixwrite=30 \
> --norandommap --randrepeat=0 -ioengine=mmap -bs=4k -numjobs=1024 -runtime=3600 \
> --time_based -group_reporting -name=mytest
>
> (that is, one hour runtime)
>
> - read: IOPS=14.0M, BW=53.4GiB/s (57.3GB/s)(188TiB/3608413msec)
> + read: IOPS=16.0M, BW=61.2GiB/s (65.7GB/s)(215TiB/3600051msec)
> - READ: bw=53.4GiB/s (57.3GB/s), 53.4GiB/s-53.4GiB/s (57.3GB/s-57.3GB/s), io=188TiB (207TB), run=3608413-3608413msec
> + READ: bw=61.2GiB/s (65.7GB/s), 61.2GiB/s-61.2GiB/s (65.7GB/s-65.7GB/s), io=215TiB (237TB), run=3600051-3600051msec
>
> Also, running Base, I see tons of:
>
> Jobs: 726 (f=726): [_(2),R(1),_(1),R(3),_(4),R(6),_(1),R(2),_(2),R(2),_(3),R(1),_(5),R(2),_(1),R(2),_(1),R(1),_(2),R(2),_(1),R(1),_(1),R(2),_(1),R(3),_(1),R(3),_(1),R(1),_(1),R(1),_(1),R(1),_(1),R(3),_(1),R(3),_(1),R(1),_(3),R(1),_(1),R(5),_(1),R(5),_(1),R(1),_(2),R(1),_(4),R(2),_(1),R(3),_(1),R(3),_(1),R(1),_(2),R(1),_(1),R(8),_(1),R(4),_(1),R(3),_(1),R(1),_(1),R(2),_(1),R(7),_(2),R(2)
>
> when the fio test terminates, which I do not see using Test. I take that as the threads do not terminate timely using the Base kernel.
>
>
> Thxs, Håkon
>
>
> [1] https://lkml.org/lkml/2024/7/3/1049
>
>
Adding Lorenzo Stoakes to Cc.