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 <size>
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.
Also, data/workload involving demotion would also be good to have for
a more complete picture.
Agree.
additionally we need to handle various cases like
- Should we choose second best target node when first node is full?
>>
Major Changes since V0:
======================
- A separate migration thread is used for migration, thus alleviating
need for
multi-threaded scanning (atleast as per tracing).
- A simple heuristic for target node calculation is added.
- prctl (David R) interface with scalar value is added to control per
task scanning.
- Steve's comment on tracing incorporated.
- Davidlohr's reported bugfix.
- Initial scan delay similar to NUMAB1 mode added.
- Got rid of migration lock during mm_walk.
PS: Occassionally I do see if scanning is too fast compared to migration,
scanning can stall waiting for lock. Should be fixed in next version by
using memslot for migration..
Disclaimer, Takeaways and discussion points and future TODOs
==============================================================
1) Source code, patch seggregation still to be improved, current
patchset only
provides a skeleton.
2) Unification of source of hotness is not easy (as mentioned perhaps
by Jonathan)
but perhaps all the consumers/producers can work coopertaively.
Scanning:
3) Major positive: Current patchset is able to cover all the process
address
space scanning effectively with simple algorithms to tune scan_size
and scan_period.
4) Effective tracking of folio's or address space using / or ideas
used in DAMON
is yet to be explored fully.
5) Use timestamp information-based migration (Similar to numab mode=2).
instead of migrating immediately when PTE A bit set.
(cons:
- It will not be accurate since it is done outside of process
context.
- Performance benefit may be lost.)
Migration:
6) Currently fast scanner can bombard migration list, need to maintain
migration list in a more
organized way (for e.g. using memslot, so that it is also helpful in
maintaining recency, frequency
information (similar to kpromoted posted by Bharata)
7) NUMAB2 throttling is very effective, we would need a common
interface to control migration
and also exploit batch migration.
Does NUMAB2 continue to exist? Are there any benefits in having two
sources?
I think there is surely a benefit in having two sources.
NUMAB2 is more accurate but slow learning.
IBS: No scan overhead but we need more sampledata.
PTE A bit: more scanning overhead (but was not much significant to
impact performance when compared with NUMAB1/NUMAB2, rather it was more
performing because of proactive migration) but has less accurate data on
hotness, target_node(?).
When system is more stable, IBS was more effective.
PTE A bit and NUMAB was effective when we needed more aggressive
migration (in that order).
- Raghu
