On 2010-08-17 23:24, Alan Cox wrote:
>> So normal mmap is ~3% slower, and prefault mmap does not seem to make
>> any measurable difference. I guess the added complexity is not really
>> worth it, for now.
>
> Do you know what fraction of this time is being spent in the kernel?
I ran 100 trials again, but now using "time -a -o logfile", so I could
run ministat over the accumulated results. This gives:
x gnugrep
+ bsdgrep-r210927 (the initial version that started this thread)
* bsdgrep-r211490 (current version)
% bsdgrep-r211490-mmap-plain
# bsdgrep-r211490-mmap-prefault
Real time:
N Min Max Median Avg Stddev
x 100 1.15 1.98 1.18 1.2122 0.11159613
+ 100 8.57 14.26 8.79 9.1823 1.0496126
* 100 2.81 6.57 2.91 3.0189 0.4304259
% 100 2.34 4.03 2.99 3.0022 0.12635992
# 100 2.85 3.49 2.88 2.8981 0.075232904
User time:
N Min Max Median Avg Stddev
x 100 0 0.07 0.03 0.0239 0.015627934
+ 100 1.6 3.33 1.9 1.976 0.30264824
* 100 0.29 1 0.39 0.4004 0.08696824
% 100 1.8 3.56 2.73 2.7274 0.13260117
# 100 2.78 3.04 2.81 2.8238 0.04039652
System time:
N Min Max Median Avg Stddev
x 100 1.08 1.91 1.15 1.1809 0.10953617
+ 100 6.55 10.9 6.94 7.1905 0.77911809
* 100 2.38 5.5 2.53 2.6061 0.35068445
% 100 0.18 0.53 0.25 0.2645 0.053586049
# 100 0.03 0.54 0.06 0.0668 0.052259647
E.g. it looks like bsdgrep with 'plain' mmap performs almost the same
as the regular bsdgrep (both around 3.0s average), but with mmap much
more of the time is spent in user mode.
And it seems prefaulting does help now! I guess it also makes sense to
add madvise(..., MADV_SEQUENTIAL)?
> Does
> the value of "sysctl vm.pmap.pde.mappings" increase as a result of your
> test? If not, there is still room for improvement in the results with
> mmap().
It always stays at 0, I have never seen any other value.
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