Documentation/admin-guide/mm/pghot.txt | 80 ++++ include/linux/migrate.h | 10 +- include/linux/mm.h | 35 +- include/linux/mmzone.h | 24 +- include/linux/pghot.h | 113 +++++ include/linux/vm_event_item.h | 5 + init/Kconfig | 13 + kernel/sched/core.c | 7 + kernel/sched/debug.c | 1 - kernel/sched/fair.c | 177 +------ kernel/sched/sched.h | 1 - mm/Kconfig | 25 + mm/Makefile | 6 + mm/huge_memory.c | 27 +- mm/memcontrol.c | 6 +- mm/memory-tiers.c | 15 +- mm/memory.c | 36 +- mm/mempolicy.c | 3 - mm/migrate.c | 88 +++- mm/mm_init.c | 10 + mm/pghot-default.c | 79 ++++ mm/pghot-precise.c | 81 ++++ mm/pghot-tunables.c | 182 ++++++++ mm/pghot.c | 618 +++++++++++++++++++++++++ mm/vmstat.c | 7 +- 25 files changed, 1411 insertions(+), 238 deletions(-) create mode 100644 Documentation/admin-guide/mm/pghot.txt create mode 100644 include/linux/pghot.h create mode 100644 mm/pghot-default.c create mode 100644 mm/pghot-precise.c create mode 100644 mm/pghot-tunables.c create mode 100644 mm/pghot.c
Hi,
This is v6 of pghot, a hot-page tracking and promotion subsystem. The
main changes in this version are to retain only hintfault as source
and a lot of cleanups, fixes and restructuring.
This patchset introduces a new subsystem for hot page tracking and
promotion (pghot) with the following goals:
- Unify hot page detection from multiple sources like hint faults,
page table scans, hardware hints (AMD IBS).
- Decouple detection from migration.
- Centralize promotion logic via per-lower-tier-node kmigrated kernel
thread.
- Move promotion rate‑limiting and related logic used by
numa_balancing=2 (NUMAB2, the current NUMA balancing–based promotion)
from the scheduler to pghot for broader reuse.
Currently, multiple kernel subsystems detect page accesses independently.
This patchset consolidates accesses from these mechanisms by providing:
- A common API for reporting page accesses.
- Shared infrastructure for tracking hotness at PFN granularity.
- Per-lower-tier-node kernel threads for promoting pages.
Here is a brief summary of how this subsystem works:
- Tracks frequency and last access time.
- Additionally, the accessing NUMA node ID (NID) for each recorded
access is also tracked in the precision mode.
- These hotness parameters are maintained in a per-PFN hotness record
within the existing mem_section data structure.
- In default mode, one byte (u8) is used for hotness record. 5 bits are
used to store time and bucketing scheme is used to represent a total
access time up to 4s with HZ=1000. Default toptier NID (0) is used as
the target for promotion which can be changed via debugfs tunable.
- In precision mode, 4 bytes (u32) are used for each hotness record.
14 bits are used to store time which can represent around 16s
with HZ=1000.
- Classifies pages as hot based on configurable thresholds.
- Pages classified as hot are marked as ready for migration using the
ready bit. Both modes use MSB of the hotness record as ready bit.
- Per-lower-tier-node kmigrated threads periodically scan the PFNs of
lower-tier nodes, checking for the migration-ready bit to perform
batched migrations. Interval between successive scans and batching
value are configurable via debugfs tunables.
Memory overhead
---------------
Default mode: 1 byte per lower-tier PFN. For a 1TB lower-tier memory
this amounts to 256MB overhead (assuming 4K pages)
Precision mode: 4 bytes per lower-tier PFN. For a 1TB of lower memory
this amounts to 1G overhead.
Bit layout of hotness record
----------------------------
Default mode
- Bits 0-1: Frequency (2bits, 4 access samples)
- Bits 2-6: Bucketed time (5bits, up to 4s with HZ=1000)
- Bit 7: Migration ready bit
Precision mode
- Bits 0-9: Target NID (10 bits)
- Bits 10-12: Frequency (3bits, 8 access samples)
- Bits 13-26: Time (14bits, up to 16s with HZ=1000)
- Bits 27-30: Reserved
- Bit 31: Migration ready bit
Potential hotness sources
-------------------------
1. NUMA Balancing (NUMAB2, Tiering mode)
2. IBS - Instruction Based Sampling, hardware based sampling
mechanism present on AMD CPUs.
3. klruscand - PTE‑A bit scanning built on MGLRU’s walk helpers.
4. folio_mark_accessed() - Page cache access tracking (unmapped
page cache pages)
Changes in v6
=============
- While earlier versions included sample implementation for all
the hotness sources listed above, I have retained only NUMAB2
in this iteration and will include the hardened versions
of them in subsequent iterations.
- Cleaned up NUMAB2 implementation by removing the unused code
(like access time tracking code etc).
- Ensured that NUMAB1 mode works as before. (Earlier versions made
NUMA hint fault handler work only for NUMAB2 version)
- NUMA Balancing tiering mode is moved to its own new config
CONFIG_NUMA_BALANCING_TIERING to make code sharing between
NUMA Balancing and pghot easier.
- A bunch of hot page promotion related stats now depend upon
CONFIG_PGHOT as the promotion engine is part of pghot.
- Fixed kmigrated to take a reference on the folio when walking
the PFNs checking for migrate-ready folios.
- Fixed speculative folio access issue reported by Chris Mason's
review-prompts.
- Added per-memcg NUMA_PAGE_MIGRATE stats accounting for batch
migration API too.
- Added support for initializing hot_maps in the newly added
sections during memory hotplug.
- Default hotness threshold window changed from 4s to 3s as the
maximum time representable in default mode is 3.9s only.
- Lot of cleanups and code restructuring.
Results
=======
Posted as replies to this mail thread.
This v6 patchset applies on top of upstream commit a989fde763f4f and
can be fetched from:
https://github.com/AMDESE/linux-mm/tree/bharata/pghot-rfcv6
v5: https://lore.kernel.org/linux-mm/20260129144043.231636-1-bharata@amd.com/
v4: https://lore.kernel.org/linux-mm/20251206101423.5004-1-bharata@amd.com/
v3: https://lore.kernel.org/linux-mm/20251110052343.208768-1-bharata@amd.com/
v2: https://lore.kernel.org/linux-mm/20250910144653.212066-1-bharata@amd.com/
v1: https://lore.kernel.org/linux-mm/20250814134826.154003-1-bharata@amd.com/
v0: https://lore.kernel.org/linux-mm/20250306054532.221138-1-bharata@amd.com/
Bharata B Rao (4):
mm: migrate: Allow misplaced migration without VMA
mm: Hot page tracking and promotion - pghot
mm: pghot: Precision mode for pghot
mm: sched: move NUMA balancing tiering promotion to pghot
Gregory Price (1):
mm: migrate: Add migrate_misplaced_folios_batch()
Documentation/admin-guide/mm/pghot.txt | 80 ++++
include/linux/migrate.h | 10 +-
include/linux/mm.h | 35 +-
include/linux/mmzone.h | 24 +-
include/linux/pghot.h | 113 +++++
include/linux/vm_event_item.h | 5 +
init/Kconfig | 13 +
kernel/sched/core.c | 7 +
kernel/sched/debug.c | 1 -
kernel/sched/fair.c | 177 +------
kernel/sched/sched.h | 1 -
mm/Kconfig | 25 +
mm/Makefile | 6 +
mm/huge_memory.c | 27 +-
mm/memcontrol.c | 6 +-
mm/memory-tiers.c | 15 +-
mm/memory.c | 36 +-
mm/mempolicy.c | 3 -
mm/migrate.c | 88 +++-
mm/mm_init.c | 10 +
mm/pghot-default.c | 79 ++++
mm/pghot-precise.c | 81 ++++
mm/pghot-tunables.c | 182 ++++++++
mm/pghot.c | 618 +++++++++++++++++++++++++
mm/vmstat.c | 7 +-
25 files changed, 1411 insertions(+), 238 deletions(-)
create mode 100644 Documentation/admin-guide/mm/pghot.txt
create mode 100644 include/linux/pghot.h
create mode 100644 mm/pghot-default.c
create mode 100644 mm/pghot-precise.c
create mode 100644 mm/pghot-tunables.c
create mode 100644 mm/pghot.c
base-commit: a989fde763f4f24209e4702f50a45be572340e68
--
2.34.1
NAS Parallel Benchmarks - BT 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 pghot case. No migrations occur.
NUMAB2 - Existing hot page promotion for the base case and
use of hint faults as source in the pghot case.
Both promotion and demotion are enabled in this case.
NUMAB3 - Enabled both regular and tiering mode of NUMA Balancing
(kernel.numa_balancing=3)
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)
NAS-BT details
--------------
Command: mpirun -np 16 /usr/bin/numactl --cpunodebind=0,1
NPB3.4.4/NPB3.4-MPI/bin/bt.F.x
While class D uses around 24G of memory (which is too less to show the benefit
of promotion), class E results in around 368G of memory which overflows my
toptier. Hence I wanted something in between these classes. So I have modified
class F to the problem size of 768 which results in around 160GB of memory.
After the memory consumption stabilizes, all the rank PIDs are paused and
their memory is moved to CXL node using migratepages command. This simulates
the situation of memory residing on lower tier node and access by BT processes
leading to promotion.
Time in seconds - Lower is better
Mop/s total - Higher is better
=====================================================================================
Base Base pghot-default
pghot-precise
NUMAB0 NUMAB2 NUMAB2 NUMAB2
=====================================================================================
Time in seconds 7321.79 4333.85 6498.78 4386.27
Mop/s total 53451.77 90303.780 60221.01 89224.51
pgpromote_success 0 41971151 423163051 41957809
pgpromote_candidate 0 0 1870949786 0
pgpromote_candidate_nrl 0 41971151 29360089 41957809
pgdemote_kswapd 0 0 391179763 0
numa_pte_updates 0 42041312 1919944389 2568923206
numa_hint_faults 0 41972330 1911683592 2562729196
=====================================================================================
pghot-default
NUMAB3
=====================================================================================
Time in seconds 4425.84
Mop/s total 88426.77
pgpromote_success 41957442
pgpromote_candidate 0
pgpromote_candidate_nrl 41957442
pgdemote_kswapd 0
numa_pte_updates 2588634775
numa_hint_faults 2581645889
=====================================================================================
- In the base case, the benchmark numbers improve significantly due to hot page
promotion.
- Though the benchmark runs for hundreds of minutes, the pages get promoted
within the first few mins.
- pghot-precise is able to match the base case numbers.
- The benchmark suffers in pghot-default case due to promotion being limited
to the default NID (0) only. This leads to excessive PTE updates, hint faults,
demotion and promotion churn.
- With NUMAB3, pghot-default case recovers the performce as in this mode
misplaced hot page migrations get right placed due to NUMA balancing
mode=1 being active.
Graph500 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 pghot case. No migrations occur.
NUMAB2 - Existing hot page promotion for the base case and
use of hint faults as source in the pghot case.
NUMAB3 - Enabled both regular and tiering mode of NUMA Balancing
(kernel.numa_balancing=3)
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)
Graph500 details
----------------
Command: mpirun -n 128 --bind-to core --map-by core
graph500/src/graph500_reference_bfs 28 16
After the graph creation, the processes are stopped and data is migrated
to CXL node 2 before continuing so that BFS phase starts accessing lower
tier memory.
Total memory usage is slightly over 100GB and will fit within Node 0 and 1.
Hence there is no memory pressure to induce demotions.
harmonic_mean_TEPS - Higher is better
=====================================================================================
Base Base pghot-default
pghot-precise
NUMAB0 NUMAB2 NUMAB2 NUMAB2
=====================================================================================
harmonic_mean_TEPS 5.07693e+08 7.08679e+08 5.56854e+08 7.39417e+08
mean_time 8.45968 6.06046 7.71283 5.80853
median_TEPS 5.08914e+08 7.23181e+08 5.51614e+08 7.58993e+08
max_TEPS 5.15226e+08 1.01654e+09 7.75233e+08 9.69136e+08
pgpromote_success 0 13797978 13746431 13752523
numa_pte_updates 0 26727341 39998363 48374479
numa_hint_faults 0 13798301 24459996 32728927
=====================================================================================
pghot-default
NUMAB3
=====================================================================================
harmonic_mean_TEPS 7.18678e+08
mean_time 5.97614
median_TEPS 7.376e+08
max_TEPS 7.47337e+08
pgpromote_success 13821625
numa_pte_updates 93534398
numa_hint_faults 69164048
=====================================================================================
- The base case shows a good improvement with NUMAB2 in harmonic_mean_TEPS.
- The same improvement gets maintained with pghot-precise too.
- pghot-default mode doesn't show benefit even when achieving similar page promotion
numbers. This mode doesn't track accessing NID and by default promotes to NID=0
which probably isn't all that beneficial as processes are running on both Node 0
and Node 1.
- pghot-default recovers the performance when balancing between toptier nodes
0 and 1 is enabled in addition to hot page promotion.
Redis-memtier 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
==============================================================
In the setup phase, 64GB database is provisioned and explicitly moved
to Node 2 by migrating redis-server's memory to Node 2.
Memtier is run on Node 1.
Parallel distribution, 50% of the keys accessed, each 4 times.
16 Threads
100 Connections per thread
77808 Requests per client
==================================================================================================
Type Ops/sec Avg. Latency p50 Latency p99 Latency p99.9
Latency KB/sec
--------------------------------------------------------------------------------------------------
Base, NUMAB0
Totals 226611.42 225.92873 224.25500 423.93500
454.65500 514886.68
--------------------------------------------------------------------------------------------------
Base, NUMAB2
Totals 257211.48 204.99755 216.06300 370.68700
454.65500 584413.47
--------------------------------------------------------------------------------------------------
pghot-default, NUMAB2
Totals 255631.78 209.20335 216.06300 378.87900
450.55900 580824.22
--------------------------------------------------------------------------------------------------
pghot-precise, NUMAB2
Totals 249494.46 209.31820 212.99100 380.92700
448.51100 566879.53
==================================================================================================
pgpromote_success
==================================
Base, NUMAB0 0
Base, NUMAB2 10,435,176
pghot-default, NUMAB2 10,435,235
pghot-precise, NUMAB2 10,435,294
==================================
- There is a clear benefit of hot page promotion seen. Both
base and pghot show similar benefits.
- The number of pages promoted in both cases are more or less
same.
==============================================================
Scenario 2 - Toptier memory overcommited, promotion + demotion
==============================================================
In the setup phase, 192GB database is provisioned. The database occupies
Node 1 entirely(~128GB) and spills over to Node 2 (~64GB).
Memtier is run on Node 1.
Parallel distribution, 50% of the keys accessed, each 4 times.
16 Threads
100 Connections per thread
233424 Requests per client
==================================================================================================
Type Ops/sec Avg. Latency p50 Latency p99 Latency p99.9
Latency KB/sec
--------------------------------------------------------------------------------------------------
Base, NUMAB0
Totals 237743.40 217.72842 201.72700 395.26300
440.31900 540389.78
--------------------------------------------------------------------------------------------------
Base, NUMAB2
Totals 235935.72 219.36544 210.94300 411.64700
477.18300 536280.93
--------------------------------------------------------------------------------------------------
pghot-default, NUMAB2
Totals 248283.99 219.74875 211.96700 413.69500
509.95100 564348.49
--------------------------------------------------------------------------------------------------
pghot-precise, NUMAB2
Totals 240529.35 222.11878 215.03900 411.64700
464.89500 546722.22
==================================================================================================
pgpromote_success pgdemote_kswapd
===============================================================
Base, NUMAB0 0 672,591
Base, NUMAB2 350,632 689,751
pghot-default, NUMAB2 17,118,987 17,421,474
pghot-precise, NUMAB2 24,030,292 24,342,569
===============================================================
- No clear benefit is seen with hot page promotion both in base and pghot case.
- Most promotion attempts in base case fail because the NUMA hint fault latency
is found to exceed the threshold value (default threshold of 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.
Because of the above difference in behaviour, more number of pages get
qualified for promotion compared to base NUMAB2.
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.
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