Yafang Shao <laoar.shao@xxxxxxxxx> writes:
> On Tue, Jul 2, 2024 at 5:10 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Yafang Shao <laoar.shao@xxxxxxxxx> writes:
>>
>> > On Tue, Jul 2, 2024 at 10:51 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>> >>
>> >> On Mon, 1 Jul 2024 22:20:46 +0800 Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>> >>
>> >> > Currently, we're encountering latency spikes in our container environment
>> >> > when a specific container with multiple Python-based tasks exits. These
>> >> > tasks may hold the zone->lock for an extended period, significantly
>> >> > impacting latency for other containers attempting to allocate memory.
>> >>
>> >> Is this locking issue well understood? Is anyone working on it? A
>> >> reasonably detailed description of the issue and a description of any
>> >> ongoing work would be helpful here.
>> >
>> > In our containerized environment, we have a specific type of container
>> > that runs 18 processes, each consuming approximately 6GB of RSS. These
>> > processes are organized as separate processes rather than threads due
>> > to the Python Global Interpreter Lock (GIL) being a bottleneck in a
>> > multi-threaded setup. Upon the exit of these containers, other
>> > containers hosted on the same machine experience significant latency
>> > spikes.
>> >
>> > Our investigation using perf tracing revealed that the root cause of
>> > these spikes is the simultaneous execution of exit_mmap() by each of
>> > the exiting processes. This concurrent access to the zone->lock
>> > results in contention, which becomes a hotspot and negatively impacts
>> > performance. The perf results clearly indicate this contention as a
>> > primary contributor to the observed latency issues.
>> >
>> > + 77.02% 0.00% uwsgi [kernel.kallsyms]
>> > [k] mmput ▒
>> > - 76.98% 0.01% uwsgi [kernel.kallsyms]
>> > [k] exit_mmap ▒
>> > - 76.97% exit_mmap
>> > ▒
>> > - 58.58% unmap_vmas
>> > ▒
>> > - 58.55% unmap_single_vma
>> > ▒
>> > - unmap_page_range
>> > ▒
>> > - 58.32% zap_pte_range
>> > ▒
>> > - 42.88% tlb_flush_mmu
>> > ▒
>> > - 42.76% free_pages_and_swap_cache
>> > ▒
>> > - 41.22% release_pages
>> > ▒
>> > - 33.29% free_unref_page_list
>> > ▒
>> > - 32.37% free_unref_page_commit
>> > ▒
>> > - 31.64% free_pcppages_bulk
>> > ▒
>> > + 28.65% _raw_spin_lock
>> > ▒
>> > 1.28% __list_del_entry_valid
>> > ▒
>> > + 3.25% folio_lruvec_lock_irqsave
>> > ▒
>> > + 0.75% __mem_cgroup_uncharge_list
>> > ▒
>> > 0.60% __mod_lruvec_state
>> > ▒
>> > 1.07% free_swap_cache
>> > ▒
>> > + 11.69% page_remove_rmap
>> > ▒
>> > 0.64% __mod_lruvec_page_state
>> > - 17.34% remove_vma
>> > ▒
>> > - 17.25% vm_area_free
>> > ▒
>> > - 17.23% kmem_cache_free
>> > ▒
>> > - 17.15% __slab_free
>> > ▒
>> > - 14.56% discard_slab
>> > ▒
>> > free_slab
>> > ▒
>> > __free_slab
>> > ▒
>> > __free_pages
>> > ▒
>> > - free_unref_page
>> > ▒
>> > - 13.50% free_unref_page_commit
>> > ▒
>> > - free_pcppages_bulk
>> > ▒
>> > + 13.44% _raw_spin_lock
>> >
>> > By enabling the mm_page_pcpu_drain() we can find the detailed stack:
>> >
>> > <...>-1540432 [224] d..3. 618048.023883: mm_page_pcpu_drain:
>> > page=0000000035a1b0b7 pfn=0x11c19c72 order=0 migratetyp
>> > e=1
>> > <...>-1540432 [224] d..3. 618048.023887: <stack trace>
>> > => free_pcppages_bulk
>> > => free_unref_page_commit
>> > => free_unref_page_list
>> > => release_pages
>> > => free_pages_and_swap_cache
>> > => tlb_flush_mmu
>> > => zap_pte_range
>> > => unmap_page_range
>> > => unmap_single_vma
>> > => unmap_vmas
>> > => exit_mmap
>> > => mmput
>> > => do_exit
>> > => do_group_exit
>> > => get_signal
>> > => arch_do_signal_or_restart
>> > => exit_to_user_mode_prepare
>> > => syscall_exit_to_user_mode
>> > => do_syscall_64
>> > => entry_SYSCALL_64_after_hwframe
>> >
>> > The servers experiencing these issues are equipped with impressive
>> > hardware specifications, including 256 CPUs and 1TB of memory, all
>> > within a single NUMA node. The zoneinfo is as follows,
>> >
>> > Node 0, zone Normal
>> > pages free 144465775
>> > boost 0
>> > min 1309270
>> > low 1636587
>> > high 1963904
>> > spanned 564133888
>> > present 296747008
>> > managed 291974346
>> > cma 0
>> > protection: (0, 0, 0, 0)
>> > ...
>> > ...
>> > pagesets
>> > cpu: 0
>> > count: 2217
>> > high: 6392
>> > batch: 63
>> > vm stats threshold: 125
>> > cpu: 1
>> > count: 4510
>> > high: 6392
>> > batch: 63
>> > vm stats threshold: 125
>> > cpu: 2
>> > count: 3059
>> > high: 6392
>> > batch: 63
>> >
>> > ...
>> >
>> > The high is around 100 times the batch size.
>> >
>> > We also traced the latency associated with the free_pcppages_bulk()
>> > function during the container exit process:
>> >
>> > 19:48:54
>> > nsecs : count distribution
>> > 0 -> 1 : 0 | |
>> > 2 -> 3 : 0 | |
>> > 4 -> 7 : 0 | |
>> > 8 -> 15 : 0 | |
>> > 16 -> 31 : 0 | |
>> > 32 -> 63 : 0 | |
>> > 64 -> 127 : 0 | |
>> > 128 -> 255 : 0 | |
>> > 256 -> 511 : 148 |***************** |
>> > 512 -> 1023 : 334 |****************************************|
>> > 1024 -> 2047 : 33 |*** |
>> > 2048 -> 4095 : 5 | |
>> > 4096 -> 8191 : 7 | |
>> > 8192 -> 16383 : 12 |* |
>> > 16384 -> 32767 : 30 |*** |
>> > 32768 -> 65535 : 21 |** |
>> > 65536 -> 131071 : 15 |* |
>> > 131072 -> 262143 : 27 |*** |
>> > 262144 -> 524287 : 84 |********** |
>> > 524288 -> 1048575 : 203 |************************ |
>> > 1048576 -> 2097151 : 284 |********************************** |
>> > 2097152 -> 4194303 : 327 |*************************************** |
>> > 4194304 -> 8388607 : 215 |************************* |
>> > 8388608 -> 16777215 : 116 |************* |
>> > 16777216 -> 33554431 : 47 |***** |
>> > 33554432 -> 67108863 : 8 | |
>> > 67108864 -> 134217727 : 3 | |
>> >
>> > avg = 3066311 nsecs, total: 5887317501 nsecs, count: 1920
>> >
>> > The latency can reach tens of milliseconds.
>> >
>> > By adjusting the vm.percpu_pagelist_high_fraction parameter to set the
>> > minimum pagelist high at 4 times the batch size, we were able to
>> > significantly reduce the latency associated with the
>> > free_pcppages_bulk() function during container exits.:
>> >
>> > nsecs : count distribution
>> > 0 -> 1 : 0 | |
>> > 2 -> 3 : 0 | |
>> > 4 -> 7 : 0 | |
>> > 8 -> 15 : 0 | |
>> > 16 -> 31 : 0 | |
>> > 32 -> 63 : 0 | |
>> > 64 -> 127 : 0 | |
>> > 128 -> 255 : 120 | |
>> > 256 -> 511 : 365 |* |
>> > 512 -> 1023 : 201 | |
>> > 1024 -> 2047 : 103 | |
>> > 2048 -> 4095 : 84 | |
>> > 4096 -> 8191 : 87 | |
>> > 8192 -> 16383 : 4777 |************** |
>> > 16384 -> 32767 : 10572 |******************************* |
>> > 32768 -> 65535 : 13544 |****************************************|
>> > 65536 -> 131071 : 12723 |************************************* |
>> > 131072 -> 262143 : 8604 |************************* |
>> > 262144 -> 524287 : 3659 |********** |
>> > 524288 -> 1048575 : 921 |** |
>> > 1048576 -> 2097151 : 122 | |
>> > 2097152 -> 4194303 : 5 | |
>> >
>> > avg = 103814 nsecs, total: 5805802787 nsecs, count: 55925
>> >
>> > After successfully tuning the vm.percpu_pagelist_high_fraction sysctl
>> > knob to set the minimum pagelist high at a level that effectively
>> > mitigated latency issues, we observed that other containers were no
>> > longer experiencing similar complaints. As a result, we decided to
>> > implement this tuning as a permanent workaround and have deployed it
>> > across all clusters of servers where these containers may be deployed.
>>
>> Thanks for your detailed data.
>>
>> IIUC, the latency of free_pcppages_bulk() during process exiting
>> shouldn't be a problem?
>
> Right. The problem arises when the process holds the lock for too
> long, causing other processes that are attempting to allocate memory
> to experience delays or wait times.
>
>> Because users care more about the total time of
>> process exiting, that is, throughput. And I suspect that the zone->lock
>> contention and page allocating/freeing throughput will be worse with
>> your configuration?
>
> While reducing throughput may not be a significant concern due to the
> minimal difference, the potential for latency spikes, a crucial metric
> for assessing system stability, is of greater concern to users. Higher
> latency can lead to request errors, impacting the user experience.
> Therefore, maintaining stability, even at the cost of slightly lower
> throughput, is preferable to experiencing higher throughput with
> unstable performance.
>
>>
>> But the latency of free_pcppages_bulk() and page allocation in other
>> processes is a problem. And your configuration can help it.
>>
>> Another choice is to change CONFIG_PCP_BATCH_SCALE_MAX. In that way,
>> you have a normal PCP size (high) but smaller PCP batch. I guess that
>> may help both latency and throughput in your system. Could you give it
>> a try?
>
> Currently, our kernel does not include the CONFIG_PCP_BATCH_SCALE_MAX
> configuration option. However, I've observed your recent improvements
> to the zone->lock mechanism, particularly commit 52166607ecc9 ("mm:
> restrict the pcp batch scale factor to avoid too long latency"), which
> has prompted me to experiment with manually setting the
> pcp->free_factor to zero. While this adjustment provided some
> improvement, the results were not as significant as I had hoped.
>
> BTW, perhaps we should consider the implementation of a sysctl knob as
> an alternative to CONFIG_PCP_BATCH_SCALE_MAX? This would allow users
> to more easily adjust it.
If you cannot test upstream behavior, it's hard to make changes to
upstream. Could you find a way to do that?
IIUC, PCP high will not influence allocate/free latency, PCP batch will.
Your configuration will influence PCP batch via configuration PCP high.
So, it may be reasonable to find a way to adjust PCP batch directly.
But, we need practical requirements and test methods first.
[snip]
--
Best Regards,
Huang, Ying
