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U2FsdGVkX1+p6VOCcuDogveTN+oeG/fz6OYeUdiCSpnyUZBItZQ99kXhSHcSHmT7gRx57IX+zPDj1lytxzqWSf4/PVHqnkiBtaNvUDvX9LFDhXDf+p1tx1uWL4QgPpBwVslT//Dgw9IIgqnZErVq0XTnie/RUDGI8i68I3+7APfEVNqUhuLzdsiVyAMCxKyh05QntgjnRuC0/hVtNG8SYx/RUTk/LHDK5b+x38Y7268R9yLRPlUzp1yHuOYZMct7ea2gbpkPbW/uTR4mHDCMKdHr3twSrzHLTej4NCMQOKTR8zf6yv+K29sYJashsyFm7S8IdN01HH9CoaNNlWKblDLUSgthZ59gwJWqlcHAq2YBQ7ZSw+jgtwxRKe4/+ghRN0yVXa9bSKL63vhF3dKzaYTfqejfotBWlBAd2J0TyrT0e2bo4OLD2twKI/o2QyfwcloN1I+/invG/NKt/dYXzS5T0QJvDrp2HhLnby3vPxlQbpacuF/6PXZcdZERhGTtYuMsbUyVgF7WGLBPAvfCRnZlIfObtmmPaXn4DrwBL2CPpItNuLiYFnS7X/LISSMU6j8kRUOFDdl8W0XSZC3YiM/bwE1Tfk8AXCee7bcGCAX4/w+vRAavS9x7+QLLPPQLHHWa5gGdwu+yLydebKsT0rWM6MBgQv1C2Ps5IhaUlGNLWfdztikG3e7+gAAzj9+wxW5CklGGzzkxb2CpmACbQ+qh1woJIf0rjROm++IRFCsnX/Z+U/krwfxsdnfLlhQ7441JA9y6znXv3RxRJSqOoR+Jak/vCxHTfgXjs7s0ppjY1nmt3pLpKqA5jeApAAYooNQlwKLR6X/J+8JiutQadgR8tPOfPK5TlLhE006luy0Zn9mKim3dvzHZc1IQa2+M1rKdqBrS8z6YcA8yIqTK3ClVvO1s+ZmHitKSQFGZqInnFaQhByAbeWn+HPBP1FMPQlsFfeAq39Ye6g8ITmb z7e7zS3s LwRke2Cz8ZX+Uib+rrns1lQxhcVPy+YfrojwnN1IfmlMABh0crEWOyZgd++JQYgSU6dLzBajuK4k4k5pWQVpFttUQZqwZA9LVb1UkOQbLI7uJH4L5qmYA+yzzVJZevZjCzeEiuCZEqwo/OjBOH1JBKyMaUOk/jz9/KwJXK14daTx7s4MdcPY2lpgfBSGEQIZxFQChG4i3sEAifk+7yUMk1Tswz34q7EZBZqQ3 X-Bogosity: Ham, tests=bogofilter, spamicity=0.003034, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/20/2025 2:21 PM, Raghavendra K T wrote: > On 3/20/2025 4:30 AM, Davidlohr Bueso wrote: >> On Wed, 19 Mar 2025, Raghavendra K T wrote: >> >>> Introduction: >>> ============= >>> In the current hot page promotion, all the activities including the >>> process address space scanning, NUMA hint fault handling and page >>> migration is performed in the process context. i.e., scanning >>> overhead is >>> borne by applications. >>> >>> This is RFC V1 patch series to do (slow tier) CXL page promotion. >>> The approach in this patchset assists/addresses the issue by adding PTE >>> Accessed bit scanning. >>> >>> Scanning is done by a global kernel thread which routinely scans all >>> the processes' address spaces and checks for accesses by reading the >>> PTE A bit. >>> >>> A separate migration thread migrates/promotes the pages to the toptier >>> node based on a simple heuristic that uses toptier scan/access >>> information >>> of the mm. >>> >>> Additionally based on the feedback for RFC V0 [4], a prctl knob with >>> a scalar value is provided to control per task scanning. >>> >>> Initial results show promising number on a microbenchmark. Soon >>> will get numbers with real benchmarks and findings (tunings). >>> >>> Experiment: >>> ============ >>> Abench microbenchmark, >>> - Allocates 8GB/16GB/32GB/64GB of memory on CXL node >>> - 64 threads created, and each thread randomly accesses pages in 4K >>>  granularity. >>> - 512 iterations with a delay of 1 us between two successive iterations. >>> >>> SUT: 512 CPU, 2 node 256GB, AMD EPYC. >>> >>> 3 runs, command:  abench -m 2 -d 1 -i 512 -s >>> >>> Calculates how much time is taken to complete the task, lower is better. >>> Expectation is CXL node memory is expected to be migrated as fast as >>> possible. >>> >>> Base case: 6.14-rc6    w/ numab mode = 2 (hot page promotion is >>> enabled). >>> patched case: 6.14-rc6 w/ numab mode = 1 (numa balancing is enabled). >>> we expect daemon to do page promotion. >>> >>> Result: >>> ======== >>>         base NUMAB2                    patched NUMAB1 >>>         time in sec  (%stdev)   time in sec  (%stdev)     %gain >>> 8GB     134.33       ( 0.19 )        120.52  ( 0.21 )     10.28 >>> 16GB     292.24       ( 0.60 )        275.97  ( 0.18 )      5.56 >>> 32GB     585.06       ( 0.24 )        546.49  ( 0.35 )      6.59 >>> 64GB    1278.98       ( 0.27 )       1205.20  ( 2.29 )      5.76 >>> >>> Base case: 6.14-rc6    w/ numab mode = 1 (numa balancing is enabled). >>> patched case: 6.14-rc6 w/ numab mode = 1 (numa balancing is enabled). >>>         base NUMAB1                    patched NUMAB1 >>>         time in sec  (%stdev)   time in sec  (%stdev)     %gain >>> 8GB     186.71       ( 0.99 )        120.52  ( 0.21 )     35.45 >>> 16GB     376.09       ( 0.46 )        275.97  ( 0.18 )     26.62 >>> 32GB     744.37       ( 0.71 )        546.49  ( 0.35 )     26.58 >>> 64GB    1534.49       ( 0.09 )       1205.20  ( 2.29 )     21.45 >> >> Very promising, but a few things. A more fair comparison would be >> vs kpromoted using the PROT_NONE of NUMAB2. Essentially disregarding >> the asynchronous migration, and effectively measuring synchronous >> vs asynchronous scanning overhead and implied semantics. Essentially >> save the extra kthread and only have a per-NUMA node migrator, which >> is the common denominator for all these sources of hotness. > > > Yes, I agree that fair comparison would be > 1) kmmscand generating data on pages to be promoted working with > kpromoted asynchronously migrating > VS > 2) NUMAB2 generating data on pages to be migrated integrated with > kpromoted. > > As Bharata already mentioned, we tried integrating kpromoted with > kmmscand generated migration list, But kmmscand generates huge amount of > scanned page data, and need to be organized better so that kpromted can > handle the migration effectively. > > (2) We have not tried it yet, will get back on the possibility (and also > numbers when both are ready). > >> >> Similarly, while I don't see any users disabling NUMAB1 _and_ enabling >> this sort of thing, it would be useful to have data on no numa balancing >> at all. If nothing else, that would measure the effects of the dest >> node heuristics. > > Last time when I checked, with patch, numbers with NUMAB=0 and NUMAB=1 > was not making much difference in 8GB case because most of the migration > was handled by kmmscand. It is because before NUMAB=1 learns and tries > to migrate, kmmscand would have already migrated. > > But a longer running/ more memory workload may make more difference. > I will comeback with that number. base NUMAB=2 Patched NUMAB=0 time in sec time in sec =================================================== 8G: 134.33 (0.19) 119.88 ( 0.25) 16G: 292.24 (0.60) 325.06 (11.11) 32G: 585.06 (0.24) 546.15 ( 0.50) 64G: 1278.98 (0.27) 1221.41 ( 1.54) We can see that numbers have not changed much between NUMAB=1 NUMAB=0 in patched case. PS: for 16G there was a bad case where a rare contention happen for lock for same mm. that we can see from stdev, which should be taken care in next version. [...]