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No migrations occur. NUMAB2 - Existing hot page promotion for the base case and use of hint faults as source in the patched case. Pghot by default promotes after two accesses but for NUMAB2 source, promotion is done after one access to match the base behaviour. (/sys/kernel/debug/pghot/freq_threshold=1) ============================================================== Scenario 1 - Enough memory in toptier and hence only promotion ============================================================== Multi-threaded application with 64 threads that access memory at 4K granularity repetitively and randomly. The number of accesses per thread and the randomness pattern for each thread are fixed beforehand. The accesses are divided into stores and loads in the ratio of 50:50. Benchmark threads run on Node 0, while memory is initially provisioned on CXL node 2 before the accesses start. Repetitive accesses results in lowertier pages becoming hot and kmigrated detecting and migrating them. The benchmark score is the time taken to finish the accesses in microseconds. The sooner it finishes the better it is. All the numbers shown below are average of 3 runs. Default mode - Time taken (microseconds, lower is better) --------------------------------------------------------- Source Base Pghot --------------------------------------------------------- NUMAB0 119,658,562 118,037,791 NUMAB2 104,205,571 102,705,330 --------------------------------------------------------- Default mode - Pages migrated (pgpromote_success) --------------------------------------------------------- Source Base Pghot --------------------------------------------------------- NUMAB0 0 0 NUMAB2 2097152 2097152 --------------------------------------------------------- Precision mode - Time taken (microseconds, lower is better) ----------------------------------------------------------- Source Base Pghot ----------------------------------------------------------- NUMAB0 119,658,562 115,173,151 NUMAB2 104,205,571 102,194,435 ----------------------------------------------------------- Precision mode - Pages migrated (pgpromote_success) --------------------------------------------------- Source Base Pghot --------------------------------------------------- NUMAB0 0 0 NUMAB2 2097152 2097152 --------------------------------------------------- Rate of migration (pgpromote_success) ----------------------------------------- Time(s) Base Pghot ----------------------------------------- 0 0 0 28 0 0 32 262144 262144 36 524288 469012 40 786432 720896 44 1048576 983040 48 1310720 1245184 52 1572864 1507328 56 1835008 1769472 60 2097152 2031616 64 2097152 2097152 ----------------------------------------- ============================================================== Scenario 2 - Toptier memory overcommited, promotion + demotion ============================================================== Single threaded application that allocates memory on both DRAM and CXL nodes using mmap(MAP_POPULATE). Every 1G region of allocated memory on CXL node is accessed at 4K granularity randomly and repetitively to build up the notion of hotness in the 1GB region that is under access. This should drive promotion. For promotion to work successfully, the DRAM memory that has been provisioned (and not being accessed) should be demoted first. There is enough free memory in the CXL node to for demotions. In summary, this benchmark creates a memory pressure on DRAM node and does CXL memory accesses to drive both demotion and promotion. The number of accesses are fixed and hence, the quicker the accessed pages get promoted to DRAM, the sooner the benchmark is expected to finish. All the numbers shown below are average of 3 runs. DRAM-node = 1 CXL-node = 2 Initial DRAM alloc ratio = 75% Allocation-size = 171798691840 Initial DRAM Alloc-size = 128849018880 Initial CXL Alloc-size = 42949672960 Hot-region-size = 1073741824 Nr-regions = 160 Nr-regions DRAM = 120 (provisioned but not accessed) Nr-hot-regions CXL = 40 Access pattern = random Access granularity = 4096 Delay b/n accesses = 0 Load/store ratio = 50l50s THP used = no Nr accesses = 42949672960 Nr repetitions = 1024 Default mode - Time taken (microseconds, lower is better) ------------------------------------------------------ Source Base Pghot ------------------------------------------------------ NUMAB0 61,028,534 59,432,137 NUMAB2 63,070,998 61,375,763 ------------------------------------------------------ Default mode - Pages migrated (pgpromote_success) ------------------------------------------------- Source Base Pghot ------------------------------------------------- NUMAB0 0 0 NUMAB2 26546 1070842 (High R2R variation in Base) --------------------------------------- Precision mode - Time taken (microseconds, lower is better) ------------------------------------------------------ Source Base Pghot ------------------------------------------------------ NUMAB0 61,028,534 60,354,547 NUMAB2 63,070,998 60,199,147 ------------------------------------------------------ Precision mode - Pages migrated (pgpromote_success) --------------------------------------------------- Source Base Pghot --------------------------------------------------- NUMAB0 0 0 NUMAB2 26546 1088621 (High R2R variation in Base) --------------------------------------------------- - The base case itself doesn't show any improvement in benchmark numbers due to hot page promotion. The same pattern is seen in pghot case with all the sources except hwhints. The benchmark itself may need tuning so that promotion helps. - There is a high run to run variation in the number of pages promoted in base case. - Most promotion attempts in base case fail because the NUMA hint fault latency is found to exceed the threshold value (default threshold is 1000ms) in majority of the promotion attempts. - Unlike base NUMAB2 where the hint fault latency is the difference between the PTE update time (during scanning) and the access time (hint fault), pghot uses a single latency threshold (3000ms in pghot-default and 5000ms in pghot-precise) for two purposes. 1. If the time difference between successive accesses are within the threshold, the page is marked as hot. 2. Later when kmigrated picks up the page for migration, it will migrate only if the difference between the current time and the time when the page was marked hot is with the threshold.