Messages in this thread | | | Date | Mon, 26 Feb 2024 16:46:00 +0800 | Subject | Re: [PATCH v2] filemap: avoid unnecessary major faults in filemap_fault() | From | "zhangpeng (AS)" <> |
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On 2024/2/26 16:20, David Hildenbrand wrote:
> On 26.02.24 08:52, Huang, Ying wrote: >> "zhangpeng (AS)" <zhangpeng362@huawei.com> writes: >> >>> On 2024/2/26 14:04, Huang, Ying wrote: >>> >>>> "zhangpeng (AS)" <zhangpeng362@huawei.com> writes: >>>> >>>>> On 2024/2/7 10:21, Huang, Ying wrote: >>>>> >>>>>> Peng Zhang <zhangpeng362@huawei.com> writes: >>>>>>> From: ZhangPeng <zhangpeng362@huawei.com> >>>>>>> >>>>>>> The major fault occurred when using mlockall(MCL_CURRENT | >>>>>>> MCL_FUTURE) >>>>>>> in application, which leading to an unexpected performance >>>>>>> issue[1]. >>>>>>> >>>>>>> This caused by temporarily cleared PTE during a >>>>>>> read+clear/modify/write >>>>>>> update of the PTE, eg, do_numa_page()/change_pte_range(). >>>>>>> >>>>>>> For the data segment of the user-mode program, the global >>>>>>> variable area >>>>>>> is a private mapping. After the pagecache is loaded, the private >>>>>>> anonymous >>>>>>> page is generated after the COW is triggered. Mlockall can lock >>>>>>> COW pages >>>>>>> (anonymous pages), but the original file pages cannot be locked >>>>>>> and may >>>>>>> be reclaimed. If the global variable (private anon page) is >>>>>>> accessed when >>>>>>> vmf->pte is zeroed in numa fault, a file page fault will be >>>>>>> triggered. >>>>>>> >>>>>>> At this time, the original private file page may have been >>>>>>> reclaimed. >>>>>>> If the page cache is not available at this time, a major fault >>>>>>> will be >>>>>>> triggered and the file will be read, causing additional overhead. >>>>>>> >>>>>>> Fix this by rechecking the PTE without acquiring PTL in >>>>>>> filemap_fault() >>>>>>> before triggering a major fault. >>>>>>> >>>>>>> Testing file anonymous page read and write page fault >>>>>>> performance in ext4 >>>>>>> and ramdisk using will-it-scale[2] on a x86 physical machine. >>>>>>> The data >>>>>>> is the average change compared with the mainline after the patch is >>>>>>> applied. The test results are within the range of fluctuation, >>>>>>> and there >>>>>>> is no obvious difference. The test results are as follows: >>>>>> You still claim that there's no difference in the test results. >>>>>> If so, >>>>>> why do you implement the patch? IMHO, you need to prove your >>>>>> patch can >>>>>> improve the performance in some cases. >>>>> I'm sorry that maybe I didn't express myself clearly. >>>>> >>>>> The purpose of this patch is to fix the issue that major fault may >>>>> still be triggered >>>>> with mlockall(), thereby improving a little performance. This >>>>> patch is more of a bugfix >>>>> than a performance improvement patch. >>>>> >>>>> This issue affects our traffic analysis service. The inbound >>>>> traffic is heavy. If a major >>>>> fault occurs, the I/O schedule is triggered and the original I/O >>>>> is suspended. Generally, >>>>> the I/O schedule is 0.7 ms. If other applications are operating >>>>> disks, the system needs >>>>> to wait for more than 10 ms. However, the inbound traffic is heavy >>>>> and the NIC buffer is >>>>> small. As a result, packet loss occurs. The traffic analysis >>>>> service can't tolerate packet >>>>> loss. >>>>> >>>>> To prevent packet loss, we use the mlockall() function to prevent >>>>> I/O. It is unreasonable >>>>> that major faults will still be triggered after mlockall() is used. >>>>> >>>>> In our service test environment, the baseline is 7 major faults/12 >>>>> hours. After applied the >>>>> unlock patch, the probability of triggering the major fault is 1 >>>>> major faults/12 hours. After >>>>> applied the lock patch, no major fault will be triggered. So only >>>>> the locked patch can actually >>>>> solve our problem. >>>> This is the data I asked for. >>>> >>>> But, you said that this is a feature bug fix instead of performance >>>> improvement. So, I checked the mlock(2), and found the following >>>> words, >>>> >>>> " >>>> mlockall() locks all pages mapped into the address space >>>> of the calling >>>> process. This includes the pages of the code, data, and >>>> stack segment, >>>> as well as shared libraries, user space kernel data, >>>> shared memory, and >>>> memory-mapped files. All mapped pages are guaranteed to >>>> be resident in >>>> RAM when the call returns successfully; the pages are >>>> guaranteed to >>>> stay in RAM until later unlocked. >>>> " >>>> >>>> In theory, the locked page are in RAM. So, IIUC, we don't violate the >>>> ABI. But, in effect, we does do that. >>> >>> "mlockall() locks all pages mapped into the address space of the >>> calling process." >>> For a private mapping, mlockall() can lock COW pages (anonymous >>> pages), but the >>> original file pages can't be locked. Maybe, we violate the ABI here. >> >> If so, please make it explicit and loudly. >> >>> This is also >>> the cause of this issue. The patch fix the impact of this issue: >>> prevent major >>> faults, reduce IO, and fix the service packet loss issue. >>> >>> Preventing major faults, and thus reducing IO, could be an important >>> reason to use >>> mlockall(). Could we fix this with the locked patch? Or is there >>> another way? >> >> Unfortunately, locked patch cause performance regressions for more >> common cases. Is it possible for us to change ptep_modify_prot_start() >> to use some magic PTE value instead of 0? That may be possible. But, >> that isn't enough, you need to change all ptep_get_and_clear() users. > > Trigger (false) major faults for mlocked memory is suboptimal. > > Having such pages temporarily not mapped (e.g., page migration) is > acceptable (pages are in RAM but are getting moved). We handle that > using nonswap migration entries. > > Let me understand the issue first: > > 1) MAP_PRIVATE file mapping that is mlocked. > > 2) We caused COW, so we now have an anonymous page mapped. That anon > page is mlocked. > > 3) Change of protection (under PT lock) will temporarily clear the PTE > > 4) Page fault will trigger and find the PTE still cleared (without PT > lock) > > 5) We don't realize that there is a page mapped and, therefore, trigger > a major fault. > > Using the PT lock would fix it properly. Doing it as in this patch can > only be considered an optimization, not a proper fix. > > Using a magic PTE to work around "just use the PT lock like everyone > else" feels a bit odd. The patch states "We don't hold PTL here as > acquiring PTL hurts performance" -- do we have any numbers on that? > Testing file anonymous page read and write page fault performance in ext4 , tmpfs and ramdisk using will-it-scale[2] on a x86 physical machine. The data is the average change compared with the mainline after the patch is applied.
with the locked patch: processes processes_idle threads threads_idle ext4 private file write: -0.51% 0.08% -0.03% -0.04% ext4 shared file write: 0.135% -0.531% 2.883% -0.772% ramdisk private file write: -0.48% 0.23% -1.08% 0.27% ramdisk private file read: 0.07% -6.90% -5.85% -0.70% tmpfs private file write: -0.344% -0.110% 0.200% 0.145% tmpfs shared file write: 0.958% 0.101% 2.781% -0.337% tmpfs private file read: -0.16% 0.00% -0.12% 0.41%
with the no locked patch: processes processes_idle threads threads_idle ext4 private file write: -1.14% -0.08% -1.87% 0.13% ext4 private file read: 0.03% -0.65% -0.51% -0.08% ramdisk private file write: -1.21% -0.21% -1.12% 0.11% ramdisk private file read: 0.00% -0.68% -0.33% -0.02%
I could also run other tests if needed.
> We could special-case that for MLOCK'ed VMAs with MCL_FUTURE, meaning, > take the PTL to double-check only in such VMAs. > Agreed. I think this solution is great. Thanks for your suggestion!
-- Best Regards, Peng
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