[V8,0/2] Currently used jhash are slow enough and replace it allow as to make KSM

Message

Timofey Titovets Sept. 13, 2018, 9:41 p.m. UTC

About speed (in kernel):
        ksm: crc32c   hash() 12081 MB/s
        ksm: xxh64    hash()  8770 MB/s
        ksm: xxh32    hash()  4529 MB/s
        ksm: jhash2   hash()  1569 MB/s

By sioh Lee tests (copy from other mail):
Test platform: openstack cloud platform (NEWTON version)
Experiment node: openstack based cloud compute node (CPU: xeon E5-2620 v3, memory 64gb)
VM: (2 VCPU, RAM 4GB, DISK 20GB) * 4
Linux kernel: 4.14 (latest version)
KSM setup - sleep_millisecs: 200ms, pages_to_scan: 200

Experiment process
Firstly, we turn off KSM and launch 4 VMs.
Then we turn on the KSM and measure the checksum computation time until full_scans become two.

The experimental results (the experimental value is the average of the measured values)
crc32c_intel: 1084.10ns
crc32c (no hardware acceleration): 7012.51ns
xxhash32: 2227.75ns
xxhash64: 1413.16ns
jhash2: 5128.30ns

In summary, the result shows that crc32c_intel has advantages over all 
of the hash function used in the experiment. (decreased by 84.54% compared to crc32c,
78.86% compared to jhash2, 51.33% xxhash32, 23.28% compared to xxhash64)
the results are similar to those of Timofey.

But,
use only xxhash for now, because for using crc32c,
cryptoapi must be initialized first - that require some
tricky solution to work good in all situations.

So:
  - Fisrt patch implement compile time pickup of fastest implementation of xxhash
    for target platform.
  - Second replace jhash2 with xxhash

Thanks.

CC: Andrea Arcangeli <[e-mail]>
CC: [e-mail]
CC: [e-mail]
CC: leesioh <[e-mail]>

Timofey Titovets (2):
  xxHash: create arch dependent 32/64-bit xxhash()
  ksm: replace jhash2 with xxhash

 include/linux/xxhash.h | 23 +++++++++++++++++++++++
 mm/Kconfig             |  1 +
 mm/ksm.c               |  4 ++--
 3 files changed, 26 insertions(+), 2 deletions(-)