Date: Tue, 27 Feb 2024 15:51:20 -0800
From: SeongJae Park <sj@...nel.org>
To: Honggyu Kim <honggyu.kim@...com>
Cc: sj@...nel.org,
	damon@...ts.linux.dev,
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Subject: Re: [RFC PATCH v2 0/7] DAMON based 2-tier memory management for CXL memory

On Mon, 26 Feb 2024 23:05:46 +0900 Honggyu Kim <honggyu.kim@...com> wrote:

> There was an RFC IDEA "DAMOS-based Tiered-Memory Management" previously
> posted at [1].
> 
> It says there is no implementation of the demote/promote DAMOS action
> are made.  This RFC is about its implementation for physical address
> space.
> 
> 
> Introduction
> ============
> 
> With the advent of CXL/PCIe attached DRAM, which will be called simply
> as CXL memory in this cover letter, some systems are becoming more
> heterogeneous having memory systems with different latency and bandwidth
> characteristics.  They are usually handled as different NUMA nodes in
> separate memory tiers and CXL memory is used as slow tiers because of
> its protocol overhead compared to local DRAM.
> 
> In this kind of systems, we need to be careful placing memory pages on
> proper NUMA nodes based on the memory access frequency.  Otherwise, some
> frequently accessed pages might reside on slow tiers and it makes
> performance degradation unexpectedly.  Moreover, the memory access
> patterns can be changed at runtime.
> 
> To handle this problem, we need a way to monitor the memory access
> patterns and migrate pages based on their access temperature.  The
> DAMON(Data Access MONitor) framework and its DAMOS(DAMON-based Operation
> Schemes) can be useful features for monitoring and migrating pages.
> DAMOS provides multiple actions based on DAMON monitoring results and it
> can be used for proactive reclaim, which means swapping cold pages out
> with DAMOS_PAGEOUT action, but it doesn't support migration actions such
> as demotion and promotion between tiered memory nodes.
> 
> This series supports two new DAMOS actions; DAMOS_DEMOTE for demotion
> from fast tiers and DAMOS_PROMOTE for promotion from slow tiers.  This
> prevents hot pages from being stuck on slow tiers, which makes
> performance degradation and cold pages can be proactively demoted to
> slow tiers so that the system can increase the chance to allocate more
> hot pages to fast tiers.
> 
> The DAMON provides various tuning knobs but we found that the proactive
> demotion for cold pages is especially useful when the system is running
> out of memory on its fast tier nodes.
> 
> Our evaluation result shows that it reduces the performance slowdown
> compared to the default memory policy from 15~17% to 4~5% when the
> system runs under high memory pressure on its fast tier DRAM nodes.
> 
> 
> DAMON configuration
> ===================
> 
> The specific DAMON configuration doesn't have to be in the scope of this
> patch series, but some rough idea is better to be shared to explain the
> evaluation result.
> 
> The DAMON provides many knobs for fine tuning but its configuration file
> is generated by HMSDK[2].  It includes gen_config.py script that
> generates a json file with the full config of DAMON knobs and it creates
> multiple kdamonds for each NUMA node when the DAMON is enabled so that
> it can run hot/cold based migration for tiered memory.

I was feeling a bit confused from here since DAMON doesn't receive parameters
via a file.  To my understanding, the 'configuration file' means the input file
for DAMON user-space tool, damo, not DAMON.  Just a trivial thing, but making
it clear if possible could help readers in my opinion.

> 
> 
> Evaluation Workload
> ===================
> 
> The performance evaluation is done with redis[3], which is a widely used
> in-memory database and the memory access patterns are generated via
> YCSB[4].  We have measured two different workloads with zipfian and
> latest distributions but their configs are slightly modified to make
> memory usage higher and execution time longer for better evaluation.
> 
> The idea of evaluation using these demote and promote actions covers
> system-wide memory management rather than partitioning hot/cold pages of
> a single workload.  The default memory allocation policy creates pages
> to the fast tier DRAM node first, then allocates newly created pages to
> the slow tier CXL node when the DRAM node has insufficient free space.
> Once the page allocation is done then those pages never move between
> NUMA nodes.  It's not true when using numa balancing, but it is not the
> scope of this DAMON based 2-tier memory management support.
> 
> If the working set of redis can be fit fully into the DRAM node, then
> the redis will access the fast DRAM only.  Since the performance of DRAM
> only is faster than partially accessing CXL memory in slow tiers, this
> environment is not useful to evaluate this patch series.
> 
> To make pages of redis be distributed across fast DRAM node and slow
> CXL node to evaluate our demote and promote actions, we pre-allocate
> some cold memory externally using mmap and memset before launching
> redis-server.  We assumed that there are enough amount of cold memory in
> datacenters as TMO[5] and TPP[6] papers mentioned.
> 
> The evaluation sequence is as follows.
> 
> 1. Turn on DAMON with DAMOS_DEMOTE action for DRAM node and
>    DAMOS_PROMOTE action for CXL node.  It demotes cold pages on DRAM
>    node and promotes hot pages on CXL node in a regular interval.
> 2. Allocate a huge block of cold memory by calling mmap and memset at
>    the fast tier DRAM node, then make the process sleep to make the fast
>    tier has insufficient memory for redis-server.
> 3. Launch redis-server and load prebaked snapshot image, dump.rdb.  The
>    redis-server consumes 52GB of anon pages and 33GB of file pages, but
>    due to the cold memory allocated at 2, it fails allocating the entire
>    memory of redis-server on the fast tier DRAM node so it partially
>    allocates the remaining on the slow tier CXL node.  The ratio of
>    DRAM:CXL depends on the size of the pre-allocated cold memory.
> 4. Run YCSB to make zipfian or latest distribution of memory accesses to
>    redis-server, then measure its execution time when it's completed.
> 5. Repeat 4 over 50 times to measure the average execution time for each
>    run.
> 6. Increase the cold memory size then repeat goes to 2.
> 
> For each test at 4 took about a minute so repeating it 50 times almost
> took about 1 hour for each test with a specific cold memory from 440GB
> to 500GB in 10GB increments for each evaluation.  So it took about more
> than 10 hours for both zipfian and latest workloads to get the entire
> evaluation results.  Repeating the same test set multiple times doesn't
> show much difference so I think it might be enough to make the result
> reliable.
> 
> 
> Evaluation Results
> ==================
> 
> All the result values are normalized to DRAM-only execution time because
> the workload cannot be faster than DRAM-only unless the workload hits
> the bandwidth peak but our redis test doesn't go beyond the bandwidth
> limit.
> 
> So the DRAM-only execution time is the ideal result without affected by
> the gap between DRAM and CXL performance difference.  The NUMA node
> environment is as follows.
> 
>   node0 - local DRAM, 512GB with a CPU socket (fast tier)
>   node1 - disabled
>   node2 - CXL DRAM, 96GB, no CPU attached (slow tier)
> 
> The following is the result of generating zipfian distribution to
> redis-server and the numbers are averaged by 50 times of execution.
> 
>   1. YCSB zipfian distribution read only workload
>   memory pressure with cold memory on node0 with 512GB of local DRAM.
>   =============+================================================+=========
>                |       cold memory occupied by mmap and memset  |
>                |   0G  440G  450G  460G  470G  480G  490G  500G |
>   =============+================================================+=========
>   Execution time normalized to DRAM-only values                 | GEOMEAN
>   -------------+------------------------------------------------+---------
>   DRAM-only    | 1.00     -     -     -     -     -     -     - | 1.00
>   CXL-only     | 1.21     -     -     -     -     -     -     - | 1.21
>   default      |    -  1.09  1.10  1.13  1.15  1.18  1.21  1.21 | 1.15
>   DAMON 2-tier |    -  1.02  1.04  1.05  1.04  1.05  1.05  1.06 | 1.04
>   =============+================================================+=========
>   CXL usage of redis-server in GB                               | AVERAGE
>   -------------+------------------------------------------------+---------
>   DRAM-only    |  0.0     -     -     -     -     -     -     - |  0.0
>   CXL-only     | 52.6     -     -     -     -     -     -     - | 52.6
>   default      |    -  19.4  26.1  32.3  38.5  44.7  50.5  50.3 | 37.4
>   DAMON 2-tier |    -   0.1   1.6   5.2   8.0   9.1  11.8  13.6 |  7.1
>   =============+================================================+=========
> 
> Each test result is based on the exeuction environment as follows.
> 
>   DRAM-only   : redis-server uses only local DRAM memory.
>   CXL-only    : redis-server uses only CXL memory.
>   default     : default memory policy(MPOL_DEFAULT).
>                 numa balancing disabled.
>   DAMON 2-tier: DAMON enabled with DAMOS_DEMOTE for DRAM nodes and
>                 DAMOS_PROMOTE for CXL nodes.
> 
> The above result shows the "default" execution time goes up as the size
> of cold memory is increased from 440G to 500G because the more cold
> memory used, the more CXL memory is used for the target redis workload
> and this makes the execution time increase.
> 
> However, "DAMON 2-tier" result shows less slowdown because the
> DAMOS_DEMOTE action at DRAM node proactively demotes pre-allocated cold
> memory to CXL node and this free space at DRAM increases more chance to
> allocate hot or warm pages of redis-server to fast DRAM node.  Moreover,
> DEMOS_PROMOTE action at CXL node also promotes hot pages of redis-server
> to DRAM node actively.
> 
> As a result, it makes more memory of redis-server stay in DRAM node
> compared to "default" memory policy and this makes the performance
> improvement.
> 
> The following result of latest distribution workload shows similar data.
> 
>   2. YCSB latest distribution read only workload
>   memory pressure with cold memory on node0 with 512GB of local DRAM.
>   =============+================================================+=========
>                |       cold memory occupied by mmap and memset  |
>                |   0G  440G  450G  460G  470G  480G  490G  500G |
>   =============+================================================+=========
>   Execution time normalized to DRAM-only values                 | GEOMEAN
>   -------------+------------------------------------------------+---------
>   DRAM-only    | 1.00     -     -     -     -     -     -     - | 1.00
>   CXL-only     | 1.18     -     -     -     -     -     -     - | 1.18
>   default      |    -  1.16  1.15  1.17  1.18  1.16  1.18  1.15 | 1.17
>   DAMON 2-tier |    -  1.04  1.04  1.05  1.05  1.06  1.05  1.06 | 1.05
>   =============+================================================+=========
>   CXL usage of redis-server in GB                               | AVERAGE
>   -------------+------------------------------------------------+---------
>   DRAM-only    |  0.0     -     -     -     -     -     -     - |  0.0
>   CXL-only     | 52.6     -     -     -     -     -     -     - | 52.6
>   default      |    -  19.3  26.1  32.2  38.5  44.6  50.5  50.6 | 37.4
>   DAMON 2-tier |    -   1.3   3.8   7.0   4.1   9.4  12.5  16.7 |  7.8
>   =============+================================================+=========
> 
> In summary of both results, our evaluation shows that "DAMON 2-tier"
> memory management reduces the performance slowdown compared to the
> "default" memory policy from 15~17% to 4~5% when the system runs with
> high memory pressure on its fast tier DRAM nodes.
> 
> The similar evaluation was done in another machine that has 256GB of
> local DRAM and 96GB of CXL memory.  The performance slowdown is reduced
> from 20~24% for "default" to 5~7% for "DAMON 2-tier".
> 
> Having these DAMOS_DEMOTE and DAMOS_PROMOTE actions can make 2-tier
> memory systems run more efficiently under high memory pressures.

Thank you for running the tests again with the new version of the patches and
sharing the results!

> 
> Signed-off-by: Honggyu Kim <honggyu.kim@...com>
> Signed-off-by: Hyeongtak Ji <hyeongtak.ji@...com>
> Signed-off-by: Rakie Kim <rakie.kim@...com>
> 
> [1] https://lore.kernel.org/damon/20231112195602.61525-1-sj@kernel.org
> [2] https://github.com/skhynix/hmsdk
> [3] https://github.com/redis/redis/tree/7.0.0
> [4] https://github.com/brianfrankcooper/YCSB/tree/0.17.0
> [5] https://dl.acm.org/doi/10.1145/3503222.3507731
> [6] https://dl.acm.org/doi/10.1145/3582016.3582063
> 
> Changes from RFC:
>   1. Move most of implementation from mm/vmscan.c to mm/damon/paddr.c.
>   2. Simplify some functions of vmscan.c and used in paddr.c, but need
>      to be reviewed more in depth.
>   3. Refactor most functions for common usage for both promote and
>      demote actions and introduce an enum migration_mode for its control.
>   4. Add "target_nid" sysfs knob for migration destination node for both
>      promote and demote actions.
>   5. Move DAMOS_PROMOTE before DAMOS_DEMOTE and move then even above
>      DAMOS_STAT.

Thank you very much for addressing many of my comments.

> 
> Honggyu Kim (3):
>   mm/damon: refactor DAMOS_PAGEOUT with migration_mode
>   mm: make alloc_demote_folio externally invokable for migration
>   mm/damon: introduce DAMOS_DEMOTE action for demotion
> 
> Hyeongtak Ji (4):
>   mm/memory-tiers: add next_promotion_node to find promotion target
>   mm/damon: introduce DAMOS_PROMOTE action for promotion
>   mm/damon/sysfs-schemes: add target_nid on sysfs-schemes
>   mm/damon/sysfs-schemes: apply target_nid for promote and demote
>     actions

Honggyu joined DAMON Beer/Coffee/Tea Chat[1] yesterday, and we discussed about
this patchset in high level.  Sharing the summary here for open discussion.  As
also discussed on the first version of this patchset[2], we want to make single
action for general page migration with minimum changes, but would like to keep
page level access re-check.  We also agreed the previously proposed DAMOS
filter-based approach could make sense for the purpose.

Because I was anyway planning making such DAMOS filter for not only
promotion/demotion but other types of DAMOS action, I will start developing the
page level access re-check results based DAMOS filter.  Once the implementation
of the prototype is done, I will share the early implementation.  Then, Honggyu
will adjust their implementation based on the filter, and run their tests again
and share the results.

[1] https://lore.kernel.org/damon/20220810225102.124459-1-sj@kernel.org/
[2] https://lore.kernel.org/damon/20240118171756.80356-1-sj@kernel.org


Thanks,
SJ

> 
>  include/linux/damon.h          |  15 +-
>  include/linux/memory-tiers.h   |  11 ++
>  include/linux/migrate_mode.h   |   1 +
>  include/linux/vm_event_item.h  |   1 +
>  include/trace/events/migrate.h |   3 +-
>  mm/damon/core.c                |   5 +-
>  mm/damon/dbgfs.c               |   2 +-
>  mm/damon/lru_sort.c            |   3 +-
>  mm/damon/paddr.c               | 282 ++++++++++++++++++++++++++++++++-
>  mm/damon/reclaim.c             |   3 +-
>  mm/damon/sysfs-schemes.c       |  39 ++++-
>  mm/internal.h                  |   1 +
>  mm/memory-tiers.c              |  43 +++++
>  mm/vmscan.c                    |  10 +-
>  mm/vmstat.c                    |   1 +
>  15 files changed, 404 insertions(+), 16 deletions(-)
> 
> 
> base-commit: 0dd3ee31125508cd67f7e7172247f05b7fd1753a
> -- 
> 2.34.1

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