Re: [RFC PATCH v6 0/5] mm: Hot page tracking and promotion infrastructure

Bharata B Rao
2026-03-23T09:56:54Z

Microbenchmark results

Test system details
-------------------
3 node AMD Zen5 system with 2 regular NUMA nodes (0, 1) and a CXL node (2)

$ numactl -H
available: 3 nodes (0-2)
node 0 cpus: 0-95,192-287
node 0 size: 128460 MB
node 1 cpus: 96-191,288-383
node 1 size: 128893 MB
node 2 cpus:
node 2 size: 257993 MB
node distances:
node 0 1 2
0: 10 32 50
1: 32 10 60
2: 255 255 10

Hotness sources
---------------
NUMAB0 - Without NUMA Balancing in base case and with no source enabled
in the patched case. 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.