[RFC MM] speculative page fault

KAMEZAWA Hiroyuki
2009-11-13T07:38:29Z

This is just a toy patch inspied by on Christoph's mmap_sem works.
Only for my hobby, now.

Not well tested. So please look into only if you have time.

My multi-thread page fault test program shows some improvement.
But I doubt my test ;) Do you have recommended benchmarks for parallel page-faults ?

Counting # of page faults per 60sec. See page-faults. bigger is better.
Test on x86-64 8cpus.

[Before]
474441.541914 task-clock-msecs # 7.906 CPUs
10318 context-switches # 0.000 M/sec
10 CPU-migrations # 0.000 M/sec
15816787 page-faults # 0.033 M/sec
1485219138381 cycles # 3130.458 M/sec (scaled from 69.99%)
295669524399 instructions # 0.199 IPC (scaled from 79.98%)
57658291915 branches # 121.529 M/sec (scaled from 79.98%)
798567455 branch-misses # 1.385 % (scaled from 79.98%)
2458780947 cache-references # 5.182 M/sec (scaled from 20.02%)
844605496 cache-misses # 1.780 M/sec (scaled from 20.02%)

[After]
471166.582784 task-clock-msecs # 7.852 CPUs
10378 context-switches # 0.000 M/sec
10 CPU-migrations # 0.000 M/sec
37950235 page-faults # 0.081 M/sec
1463000664470 cycles # 3105.060 M/sec (scaled from 70.32%)
346531590054 instructions # 0.237 IPC (scaled from 80.20%)
63309364882 branches # 134.367 M/sec (scaled from 80.19%)
448256258 branch-misses # 0.708 % (scaled from 80.20%)
2601112130 cache-references # 5.521 M/sec (scaled from 19.81%)
872978619 cache-misses # 1.853 M/sec (scaled from 19.80%)

Main concept of this patch is
- Do page fault without taking mm->mmap_sem until some modification in vma happens.
- All page fault via get_user_pages() should have to take mmap_sem.
- find_vma()/rb_tree must be walked under proper locks. For avoiding that, use
per-thread cache.

It seems I don't have enough time to update this, more.
So, I dump patches here just for share.

Thanks,
-Kame