Hi everyone,
I took at look at the optimized straw2 code today. There was one typo
(s/i/u/) and it looks like it works as well as the previous version. I've
pushed the latest code and some additional tests to the wip-crush-straw2
branch.
I went on to do some tests of the variance in the resulting distribution
when the weights are all identical vs somewhat variable vs very skewed. I
found that for a more 'typical' distribution of weights (e.g, .5 - 3) the
std deviation stays flat. However, for very large skews, there are
interesting effects that emerge based on very small differences in the ln
value. For example, here we adjust the table value into a negative
integer in mapper.c:
ln = crush_ln(u) - 0x10000;
If I adjust that 0x10000 value only slightly, and generate a distribution
across a set of scaled weights (e.g., 1, 2, 4, 8, 16, 32), I will see the
low weights either more heavily or less heavily weighted than they should
be. 0x10000 skews them a bit low, 0xffff skews them a bit high. e.g.,
for 0x10000 I get
expect 169.664
osd weight count adjusted
0 1 102 102 <-- here
1 1.75 212 121
2 3.062 442 144
3 5.359 858 160
4 9.379 1483 158
5 16.41 2673 162
6 28.72 4756 165
7 50.27 8620 171
8 87.96 14851 168
9 153.9 26019 169
10 269.4 45674 169
11 471.4 79768 169
12 825 139949 169
13 1444 245958 170
14 2527 428635 169
std dev 88.6993
vs 12.1484 (expected)
While for 0xffff I get:
expect 169.664
osd weight count adjusted
0 1 272 272
1 1.75 395 225
2 3.062 596 194
3 5.359 1009 188
4 9.379 1665 177
5 16.41 2869 174
6 28.72 4936 171
7 50.27 8769 174
8 87.96 14981 170
9 153.9 26102 169
10 269.4 45729 169
11 471.4 79737 169
12 825 139719 169
13 1444 245486 170
14 2527 427735 169
std dev 121.243
vs 13.1205 (expected)
This could be written off to chance, but I see the same effect amplified
for 0xfffe and 0x10001.
I suppose if we have to choose I think skewing high makes more sense since
it means the high weight items won't skew high and overfill (although in
practice that doesn't seem to be happening.. it's only on the low end that
things get wonky.. the high end looks quite good).
In any case, two things:
1) Maybe someone can check my math on the stddev calculation. I'm scaling
the actual placement count by the weight (adj = count/weight) and then
doing the stddev of the adjusted values. But the expected value is always
about 1/3 or 1/4 of that. I can't tell if that's because the hash is weak
or because I'm doing something wrong with the calculation. Notably, if I
plug in rand() with equal weights for a sanity check, I still get a stddev
of 953 vs expected 249... about what I see from straw2. Strangely when
I use straw I get a bit better than taht, 476. Am I crazy? See:
./unittest_crush_wrapper --gtest_filter=CrushWrapper.straw2
2) Given how sensitive the ln value is to 0xffff vs 0x10000, I'm highly
suspicious that slightly better precision will help us out. Notably, we
do
ln = crush_ln(u) - 0x10000;
/*
* divide by 16.16 fixed-point weight
*/
draw = (ln << 32) / bucket->item_weights[i];
...so some extra bits of precision (instead of having the lower 32 zeroed)
would probably help a lot.
Can crush_ln() be modifed to provide more bits of precision without
increasing the size of the tables? I don't really understand the math
it's based on, but it looks like the same curve is being added in with
different precision or something.. and a lot of bits in LH are being
thrown out here
LH >>= (48-12);
If there were 8 more bits returned from crush_ln and we shifted by 8 fewer
bits in bucket_straw2_choose, for example, I bet we could eliminate some
of the low-weight effect I'm seeing.
To see what I'm seeing, you can run
./unittest_crush_wrapper --gtest_filter=CrushWrapper.straw2_stddev
Any input would be helpful, thanks!
sage
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