On 04/24/2014 01:45 AM, Mel Gorman wrote:
>> > +/*
>> > + * See Documentation/x86/tlb.txt for details. We choose 33
>> > + * because it is large enough to cover the vast majority (at
>> > + * least 95%) of allocations, and is small enough that we are
>> > + * confident it will not cause too much overhead. Each single
>> > + * flush is about 100 cycles, so this caps the maximum overhead
>> > + * at _about_ 3,000 cycles.
>> > + */
>> > +/* in units of pages */
>> > +unsigned long tlb_single_page_flush_ceiling = 1;
>> > +
> This comment is premature. The documentation file does not exist yet and
> 33 means nothing yet. Out of curiousity though, how confident are you
> that a TLB flush is generally 100 cycles across different generations
> and manufacturers of CPUs? I'm not suggesting you change it or auto-tune
> it, am just curious.
First of all, I changed the units here at some point, and I screwed up
the comments. I meant 100 nanoseconds, *not* cycles.
For the sake of completeness, here are the data on a Westmere CPU. I'm
not _quite_ sure why the <=5 pages cases are so slow per-page compared
to when we're flushing larger numbers of pages. (I also only printed
out the flush sizes with >100 samples):
The overall average was 151ns, and for 6 pages and up it was 107ns.
1 1560658 279861777 avg/page: 179
2 179981 85329139 avg/page: 237
3 99797 146972011 avg/page: 490
4 161470 133072233 avg/page: 206
5 44150 42142670 avg/page: 190
6 17364 12063833 avg/page: 115
7 12325 9899412 avg/page: 114
8 4202 3838077 avg/page: 114
9 811 990320 avg/page: 135
10 4448 4955283 avg/page: 111
11 69051 86723229 avg/page: 114
12 465 642204 avg/page: 115
13 157 226814 avg/page: 111
16 781 1741461 avg/page: 139
17 1506 2778201 avg/page: 108
18 110 211216 avg/page: 106
19 13322 27941893 avg/page: 110
21 1828 4092988 avg/page: 106
24 1566 4057605 avg/page: 107
25 246 646463 avg/page: 105
29 411 1275101 avg/page: 106
33 3191 11775818 avg/page: 111
52 3096 17297873 avg/page: 107
65 2244 15349445 avg/page: 105
129 2278 33246120 avg/page: 113
240 12181 305529055 avg/page: 104
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