Re: [PATCH 0/9] x86/clear_huge_page: multi-page clearing

Raghavendra K T
2023-04-05T19:50:07Z

On 4/3/2023 10:52 AM, Ankur Arora wrote:
> This series introduces multi-page clearing for hugepages.
>
> This is a follow up of some of the ideas discussed at:
> https://lore.kernel.org/lkml/CAHk-=wj9En-BC4t7J9xFZOws5ShwaR9yor7FxHZr8CTVyEP_+Q@mail.gmail.com/
>
> On x86 page clearing is typically done via string intructions. These,
> unlike a MOV loop, allow us to explicitly advertise the region-size to
> the processor, which could serve as a hint to current (and/or
> future) uarchs to elide cacheline allocation.
>
> In current generation processors, Milan (and presumably other Zen
> variants) use the hint to elide cacheline allocation (for
> region-size > LLC-size.)
>
> An additional reason for doing this is that string instructions are typically
> microcoded, and clearing in bigger chunks than the current page-at-a-
> time logic amortizes some of the cost.
>
> All uarchs tested (Milan, Icelakex, Skylakex) showed improved performance.
>
> There are, however, some problems:
>
> 1. extended zeroing periods means there's an increased latency due to
> the now missing preemption points.
>
> That's handled in patches 7, 8, 9:
> "sched: define TIF_ALLOW_RESCHED"
> "irqentry: define irqentry_exit_allow_resched()"
> "x86/clear_huge_page: make clear_contig_region() preemptible"
> by the context marking itself reschedulable, and rescheduling in
> irqexit context if needed (for PREEMPTION_NONE/_VOLUNTARY.)
>
> 2. the current page-at-a-time clearing logic does left-right narrowing
> towards the faulting page which benefits workloads by maintaining
> cache locality for workloads which have a sequential pattern. Clearing
> in large chunks loses that.
>
> Some (but not all) of that could be ameliorated by something like
> this patch:
> https://lore.kernel.org/lkml/20220606203725.1313715-1-ankur.a.arora@oracle.com/
>
> But, before doing that I'd like some comments on whether that is
> worth doing for this specific use case?
>
> Rest of the series:
> Patches 1, 2, 3:
> "huge_pages: get rid of process_huge_page()"
> "huge_page: get rid of {clear,copy}_subpage()"
> "huge_page: allow arch override for clear/copy_huge_page()"
> are mechanical and they simplify some of the current clear_huge_page()
> logic.
>
> Patches 4, 5:
> "x86/clear_page: parameterize clear_page*() to specify length"
> "x86/clear_pages: add clear_pages()"
>
> add clear_pages() and helpers.
>
> Patch 6: "mm/clear_huge_page: use multi-page clearing" adds the
> chunked x86 clear_huge_page() implementation.
>
>
> Performance
> ==
>
> Demand fault performance gets a decent boost:
>
> *Icelakex* mm/clear_huge_page x86/clear_huge_page change
> (GB/s) (GB/s)
>
> pg-sz=2MB 8.76 11.82 +34.93%
> pg-sz=1GB 8.99 12.18 +35.48%
>
>
> *Milan* mm/clear_huge_page x86/clear_huge_page change
> (GB/s) (GB/s)
>
> pg-sz=2MB 12.24 17.54 +43.30%
> pg-sz=1GB 17.98 37.24 +107.11%
>
>
> vm-scalability/case-anon-w-seq-hugetlb, gains in stime but performs
> worse when user space tries to touch those pages:
>
> *Icelakex* mm/clear_huge_page x86/clear_huge_page change
> (mem=4GB/task, tasks=128)
>
> stime 293.02 +- .49% 239.39 +- .83% -18.30%
> utime 440.11 +- .28% 508.74 +- .60% +15.59%
> wall-clock 5.96 +- .33% 6.27 +-2.23% + 5.20%
>
>
> *Milan* mm/clear_huge_page x86/clear_huge_page change
> (mem=1GB/task, tasks=512)
>
> stime 490.95 +- 3.55% 466.90 +- 4.79% - 4.89%
> utime 276.43 +- 2.85% 311.97 +- 5.15% +12.85%
> wall-clock 3.74 +- 6.41% 3.58 +- 7.82% - 4.27%
>
> Also at:
> github.com/terminus/linux clear-pages.v1
>
> Comments appreciated!
>

Hello Ankur,

Was able to test your patches. To summarize, am seeing 2x-3x perf
improvement for 2M, 1GB base hugepage sizes.

SUT: Genoa AMD EPYC
Thread(s) per core: 2
Core(s) per socket: 128
Socket(s): 2

NUMA:
NUMA node(s): 2
NUMA node0 CPU(s): 0-127,256-383
NUMA node1 CPU(s): 128-255,384-511

Test: Use mmap(MAP_HUGETLB) to demand a fault on 64GB region (NUMA
node0), for both base-hugepage-size=2M and 1GB

perf stat -r 10 -d -d numactl -m 0 -N 0 <test>

time in seconds elapsed (average of 10 runs) (lower = better)

Result:
page-size mm/clear_huge_page x86/clear_huge_page change %
2M 5.4567 2.6774 -50.93
1G 2.64452 1.011281 -61.76

Full perfstat info

page size = 2M mm/clear_huge_page

Performance counter stats for 'numactl -m 0 -N 0 map_hugetlb_2M' (10
runs):

5,434.71 msec task-clock # 0.996 CPUs
utilized ( +- 0.55% )
8 context-switches # 1.466 /sec
( +- 4.66% )
0 cpu-migrations # 0.000 /sec
32,918 page-faults # 6.034 K/sec
( +- 0.00% )
16,977,242,482 cycles # 3.112 GHz
( +- 0.04% ) (35.70%)
1,961,724 stalled-cycles-frontend # 0.01% frontend
cycles idle ( +- 1.09% ) (35.72%)
35,685,674 stalled-cycles-backend # 0.21% backend
cycles idle ( +- 3.48% ) (35.74%)
1,038,327,182 instructions # 0.06 insn per cycle
# 0.04 stalled
cycles per insn ( +- 0.38% ) (35.75%)
221,409,216 branches # 40.584 M/sec
( +- 0.36% ) (35.75%)
350,730 branch-misses # 0.16% of all
branches ( +- 1.18% ) (35.75%)
2,520,888,779 L1-dcache-loads # 462.077 M/sec
( +- 0.03% ) (35.73%)
1,094,178,209 L1-dcache-load-misses # 43.46% of all
L1-dcache accesses ( +- 0.02% ) (35.71%)
67,751,730 L1-icache-loads # 12.419 M/sec
( +- 0.11% ) (35.70%)
271,118 L1-icache-load-misses # 0.40% of all
L1-icache accesses ( +- 2.55% ) (35.70%)
506,635 dTLB-loads # 92.866 K/sec
( +- 3.31% ) (35.70%)
237,385 dTLB-load-misses # 43.64% of all
dTLB cache accesses ( +- 7.00% ) (35.69%)
268 iTLB-load-misses # 6700.00% of all
iTLB cache accesses ( +- 13.86% ) (35.70%)

5.4567 +- 0.0300 seconds time elapsed ( +- 0.55% )

page size = 2M x86/clear_huge_page
Performance counter stats for 'numactl -m 0 -N 0 map_hugetlb_2M' (10
runs):

2,780.69 msec task-clock # 1.039 CPUs
utilized ( +- 1.03% )
3 context-switches # 1.121 /sec
( +- 21.34% )
0 cpu-migrations # 0.000 /sec
32,918 page-faults # 12.301 K/sec
( +- 0.00% )
8,143,619,771 cycles # 3.043 GHz
( +- 0.25% ) (35.62%)
2,024,872 stalled-cycles-frontend # 0.02% frontend
cycles idle ( +-320.93% ) (35.66%)
717,198,728 stalled-cycles-backend # 8.82% backend
cycles idle ( +- 8.26% ) (35.69%)
606,549,334 instructions # 0.07 insn per cycle
# 1.39 stalled
cycles per insn ( +- 0.23% ) (35.73%)
108,856,550 branches # 40.677 M/sec
( +- 0.24% ) (35.76%)
202,490 branch-misses # 0.18% of all
branches ( +- 3.58% ) (35.78%)
2,348,818,806 L1-dcache-loads # 877.701 M/sec
( +- 0.03% ) (35.78%)
1,081,562,988 L1-dcache-load-misses # 46.04% of all
L1-dcache accesses ( +- 0.01% ) (35.78%)
<not supported> LLC-loads
<not supported> LLC-load-misses
43,411,167 L1-icache-loads # 16.222 M/sec
( +- 0.19% ) (35.77%)
273,042 L1-icache-load-misses # 0.64% of all
L1-icache accesses ( +- 4.94% ) (35.76%)
834,482 dTLB-loads # 311.827 K/sec
( +- 9.73% ) (35.72%)
437,343 dTLB-load-misses # 65.86% of all
dTLB cache accesses ( +- 8.56% ) (35.68%)
0 iTLB-loads # 0.000 /sec
(35.65%)
160 iTLB-load-misses # 1777.78% of all
iTLB cache accesses ( +- 15.82% ) (35.62%)

2.6774 +- 0.0287 seconds time elapsed ( +- 1.07% )

page size = 1G mm/clear_huge_page
Performance counter stats for 'numactl -m 0 -N 0 map_hugetlb_1G' (10
runs):

2,625.24 msec task-clock # 0.993 CPUs
utilized ( +- 0.23% )
4 context-switches # 1.513 /sec
( +- 4.49% )
1 cpu-migrations # 0.378 /sec
214 page-faults # 80.965 /sec
( +- 0.13% )
8,178,624,349 cycles # 3.094 GHz
( +- 0.23% ) (35.65%)
2,942,576 stalled-cycles-frontend # 0.04% frontend
cycles idle ( +- 75.22% ) (35.69%)
7,117,425 stalled-cycles-backend # 0.09% backend
cycles idle ( +- 3.79% ) (35.73%)
454,521,647 instructions # 0.06 insn per cycle
# 0.02 stalled
cycles per insn ( +- 0.10% ) (35.77%)
113,223,853 branches # 42.837 M/sec
( +- 0.08% ) (35.80%)
84,766 branch-misses # 0.07% of all
branches ( +- 5.37% ) (35.80%)
2,294,528,890 L1-dcache-loads # 868.111 M/sec
( +- 0.02% ) (35.81%)
1,075,907,551 L1-dcache-load-misses # 46.88% of all
L1-dcache accesses ( +- 0.02% ) (35.78%)
26,167,323 L1-icache-loads # 9.900 M/sec
( +- 0.24% ) (35.74%)
139,675 L1-icache-load-misses # 0.54% of all
L1-icache accesses ( +- 0.37% ) (35.70%)
3,459 dTLB-loads # 1.309 K/sec
( +- 12.75% ) (35.67%)
732 dTLB-load-misses # 19.71% of all
dTLB cache accesses ( +- 26.61% ) (35.62%)
11 iTLB-load-misses # 192.98% of all
iTLB cache accesses ( +-238.28% ) (35.62%)

2.64452 +- 0.00600 seconds time elapsed ( +- 0.23% )

page size = 1G x86/clear_huge_page
Performance counter stats for 'numactl -m 0 -N 0 map_hugetlb_1G' (10
runs):

1,009.09 msec task-clock # 0.998 CPUs
utilized ( +- 0.06% )
2 context-switches # 1.980 /sec
( +- 23.63% )
1 cpu-migrations # 0.990 /sec
214 page-faults # 211.887 /sec
( +- 0.16% )
3,154,980,463 cycles # 3.124 GHz
( +- 0.06% ) (35.77%)
145,051 stalled-cycles-frontend # 0.00% frontend
cycles idle ( +- 6.26% ) (35.78%)
730,087,143 stalled-cycles-backend # 23.12% backend
cycles idle ( +- 9.75% ) (35.78%)
45,813,391 instructions # 0.01 insn per cycle
# 18.51 stalled
cycles per insn ( +- 1.00% ) (35.78%)
8,498,282 branches # 8.414 M/sec
( +- 1.54% ) (35.78%)
63,351 branch-misses # 0.74% of all
branches ( +- 6.70% ) (35.69%)
29,135,863 L1-dcache-loads # 28.848 M/sec
( +- 5.67% ) (35.68%)
8,537,280 L1-dcache-load-misses # 28.66% of all
L1-dcache accesses ( +- 10.15% ) (35.68%)
1,040,087 L1-icache-loads # 1.030 M/sec
( +- 1.60% ) (35.68%)
9,147 L1-icache-load-misses # 0.85% of all
L1-icache accesses ( +- 6.50% ) (35.67%)
1,084 dTLB-loads # 1.073 K/sec
( +- 12.05% ) (35.68%)
431 dTLB-load-misses # 40.28% of all
dTLB cache accesses ( +- 43.46% ) (35.68%)
16 iTLB-load-misses # 0.00% of all
iTLB cache accesses ( +- 40.54% ) (35.68%)

1.011281 +- 0.000624 seconds time elapsed ( +- 0.06% )

Please feel free to add

Tested-by: Raghavendra K T <raghavendra.kt@amd.com>

Will come back with further observations on patch/performance if any

Thanks and Regards