On 2/18/2020 8:51 PM, Mark Zhang wrote:
On 2/19/2020 1:41 AM, Tom Talpey wrote:
On 2/18/2020 9:16 AM, Tom Talpey wrote:
On 2/15/2020 1:27 AM, Mark Zhang wrote:
On 2/14/2020 10:23 PM, Mark Zhang wrote:
On 2/13/2020 11:41 PM, Jason Gunthorpe wrote:
On Thu, Feb 13, 2020 at 10:26:09AM -0500, Tom Talpey wrote:
If both src & dst ports are in the high value range you loss those
hash bits in the masking.
If src & dst port are both 0xE000, your masked hash equals 0. You'll
get the same hash if both ports are equal 0xF000.
Sure, but this is because it's a 20-bit hash of a 32-bit object.
There
will always be collisions, this is just one example. My concern is
the
statistical spread of the results. I argue it's not changed by the
proposed bit-folding, possibly even damaged.
I've always thought that 'folding' by modulo results in an abnormal
statistical distribution
The point here is not collisions but to have a hash distribution which
is generally uniform for the input space.
Alex, it would be good to make a quick program to measure the
uniformity of the distribution..
Hi,
I did some tests with a quick program (hope it's not buggy...),
seems the hash without "folding" has a better distribution than hash
with fold. The "hash quality" is reflected by the "total_access"[1]
below.
I tested only with cma_dport from 18515 (ib_write_bw default) to
18524. I can do more tests if required, for example use multiple
cma_dport in one statistic.
[1]
https://stackoverflow.com/questions/24729730/measuring-a-hash-functions-quality-for-use-with-maps-assosiative-arrays
$ ./a
max: Say for slot x there are tb[x] items, then 'max = max(tb[x])';
Lower is better;
min: Say for slot x there are tb[x] items, then 'min = min(tb[x])';
Likely min is always 0
total_access: The sum of all 'accesses' (for each slot:
accesses=n*(n+1)/2); Lower is better
n[X]: How many slots that has X items
cm source port range [32768, 65534], dest port 18515:
Hash with folding:
flow_label: max 2 min 0 total_access 32766 n[1] = 32514 n[2] =
126
udp_sport: max 10 min 0 total_access 51740 n[1] = 4420 n[2] =
4670 n[3] = 3112 n[4] = 1433 n[5] = 535 n[6] = 163 n[7] = 31
n[8] = 5 n[9] = 2 n[10] = 1
Hash without folding:
flow_label: max 1 min 0 total_access 32766 n[1] = 32766
udp_sport: max 4 min 0 total_access 48618 n[1] = 532 n[2] =
7926 n[3] = 530 n[4] = 3698
cm source port range [32768, 65534], dest port 18516:
Hash with folding:
flow_label: max 3 min 0 total_access 32774 n[1] = 31214 n[2] =
770 n[3] = 4
udp_sport: max 8 min 0 total_access 50808 n[1] = 4406 n[2] =
4873 n[3] = 3157 n[4] = 1413 n[5] = 509 n[6] = 129 n[7] = 20
n[8] = 4
Hash without folding:
flow_label: max 1 min 0 total_access 32766 n[1] = 32766
udp_sport: max 2 min 1 total_access 32766 n[1] = 2 n[2] =
16382
cm source port range [32768, 65534], dest port 18517:
Hash with folding:
flow_label: max 2 min 0 total_access 32766 n[1] = 32250 n[2] =
258
udp_sport: max 10 min 0 total_access 54916 n[1] = 4536 n[2] =
4170 n[3] = 2817 n[4] = 1445 n[5] = 622 n[6] = 275 n[7] = 94
n[8] = 22 n[9] = 5 n[10] = 2
Hash without folding:
flow_label: max 1 min 0 total_access 32766 n[1] = 32766
udp_sport: max 3 min 1 total_access 38402 n[1] = 2820 n[2] =
10746 n[3] = 2818
cm source port range [32768, 65534], dest port 18518:
Hash with folding:
flow_label: max 2 min 0 total_access 32766 n[1] = 32066 n[2] =
350
udp_sport: max 8 min 0 total_access 50018 n[1] = 4435 n[2] =
4970 n[3] = 3294 n[4] = 1376 n[5] = 465 n[6] = 92 n[7] = 16
n[8] = 2
Hash without folding:
flow_label: max 1 min 0 total_access 32766 n[1] = 32766
udp_sport: max 2 min 1 total_access 32766 n[1] = 2 n[2] =
16382
cm source port range [32768, 65534], dest port 18519:
Hash with folding:
flow_label: max 3 min 0 total_access 32774 n[1] = 31816 n[2] =
469 n[3] = 4
udp_sport: max 8 min 0 total_access 51462 n[1] = 4414 n[2] =
4734 n[3] = 3088 n[4] = 1466 n[5] = 508 n[6] = 160 n[7] = 32
n[8] = 4
Hash without folding:
flow_label: max 1 min 0 total_access 32766 n[1] = 32766
udp_sport: max 4 min 0 total_access 45490 n[1] = 3662 n[2] =
6360 n[3] = 3660 n[4] = 1351
cm source port range [32768, 65534], dest port 18520:
Hash with folding:
flow_label: max 6 min 0 total_access 34618 n[1] = 20349 n[2] =
5027 n[3] = 550 n[4] = 164 n[5] = 9 n[6] = 2
udp_sport: max 13 min 0 total_access 82542 n[1] = 549 n[2] =
1167 n[3] = 1635 n[4] = 1706 n[5] = 1341 n[6] = 836 n[7] = 483
n[8] = 223 n[9] = 87 n[10] = 27
Hash without folding:
flow_label: max 1 min 0 total_access 32766 n[1] = 32766
udp_sport: max 4 min 0 total_access 65530 n[3] = 2 n[4] = 8190
cm source port range [32768, 65534], dest port 18521:
Hash with folding:
flow_label: max 2 min 0 total_access 32766 n[1] = 31924 n[2] =
421
udp_sport: max 9 min 0 total_access 51864 n[1] = 4505 n[2] =
4645 n[3] = 3038 n[4] = 1464 n[5] = 542 n[6] = 154 n[7] = 43
n[8] = 6 n[9] = 2
Hash without folding:
flow_label: max 1 min 0 total_access 32766 n[1] = 32766
udp_sport: max 3 min 1 total_access 32810 n[1] = 24 n[2] =
16338 n[3] = 22
cm source port range [32768, 65534], dest port 18522:
Hash with folding:
flow_label: max 3 min 0 total_access 32768 n[1] = 32197 n[2] =
283 n[3] = 1
udp_sport: max 9 min 0 total_access 50850 n[1] = 4561 n[2] =
4756 n[3] = 3187 n[4] = 1452 n[5] = 453 n[6] = 137 n[7] = 29
n[8] = 2 n[9] = 2
Hash without folding:
flow_label: max 1 min 0 total_access 32766 n[1] = 32766
udp_sport: max 2 min 1 total_access 32766 n[1] = 2 n[2] =
16382
cm source port range [32768, 65534], dest port 18523:
Hash with folding:
flow_label: max 2 min 0 total_access 32766 n[1] = 32514 n[2] =
126
udp_sport: max 8 min 0 total_access 52208 n[1] = 4426 n[2] =
4609 n[3] = 3069 n[4] = 1435 n[5] = 533 n[6] = 180 n[7] = 50
n[8] = 10
Hash without folding:
flow_label: max 1 min 0 total_access 32766 n[1] = 32766
udp_sport: max 4 min 0 total_access 46062 n[1] = 3096 n[2] =
6640 n[3] = 3094 n[4] = 1777
cm source port range [32768, 65534], dest port 18524:
Hash with folding:
flow_label: max 3 min 0 total_access 32774 n[1] = 31362 n[2] =
696 n[3] = 4
udp_sport: max 8 min 0 total_access 49490 n[1] = 4440 n[2] =
5148 n[3] = 3240 n[4] = 1413 n[5] = 394 n[6] = 97 n[7] = 14
n[8] = 1
Hash without folding:
flow_label: max 1 min 0 total_access 32766 n[1] = 32766
udp_sport: max 2 min 1 total_access 32766 n[1] = 2 n[2] =
16382
Another finding is, when cma_dport is multiple of 0x200 (i.e., 0x600,
0x800, ... 0xFE00), the hash distribution is tens of times worse then
others. For examples when dport is 18431 and 18432:
cm source port range [32768, 65534], dest port 18431:
Hash with folding:
flow_label: max 2 min 0 total_access 32766
udp_sport: max 8 min 0 total_access 50410
Hash without folding:
flow_label: max 1 min 0 total_access 32766
udp_sport: max 4 min 0 total_access 48126
cm source port range [32768, 65534], dest port 18432(0x4800):
Hash with folding:
flow_label: max 133 min 0 total_access 1072938
udp_sport: max 203 min 0 total_access 2126644
Hash without folding:
flow_label: max 64 min 0 total_access 1048450
udp_sport: max 1024 min 0 total_access 16775170
Good data! It certainly indicates an issue with the simple
binary modulus for treuncating 32->20 bits. But the extremely
narrow testing range limits the conclusions considerably:
>> I tested only with cma_dport from 18515 (ib_write_bw default) to
>> 18524. I can do more tests if required, for example use multiple
>> cma_dport in one statistic.
This hash is intended to provide entropy across the entire port
range and we should evaluate it as such. At a minimum, the source
port can vary much more widely, from Alex's original message it's
0xC000 - 0xFFFF.
UDP source port selection must adhere IANA port allocation ranges.
Thus we will
be using IANA recommendation for Ephemeral port range of:
49152-65535, or in
hex: 0xC000-0xFFFF.
I'm not certain what the range of the destination port might be, but
as a Service ID, a good assumption is the full range of 0x1 - 0xBFFF.
Any chance you could scale up your test, to measure the original
proposed hash across these broader ranges?
u32 hash = DstPort * SrcPort;
hash ^= (hash >> 16);
hash ^= (hash >> 8);
AH_ATTR.GRH.flow_label = hash AND IB_GRH_FLOWLABEL_MASK;
I did an even quicker-and-dirtier test, with the attached. Both
the folding and non-folding methods display, to me, pretty much
the same behavior. And there's a fairly significant periodicity
with a doubling of the hash collision rate, every 8 or so buckets.
The "folding" version has higher spikes at these points than the
non-folding, in fact. As you mentioned, there are a few more "zero"
hashes, but that's expected, and not that different for both.
Assuming you agree with my C000-FFFF and 1-BFFF port ranges, there
are 800M possible permutations, and of course 1M hash buckets. So,
an 800:1 collision rate is expected. But the numbers range from
the mid-300's to several-1000's. That variance seems high to me.
I really think there needs to be a flatter spectrum, here. These
collisions can cause significant congestion effects at scale. I
suggested trying a CRC-20 of the 32-bit src<<16|dst, but it's going
to take me a little time to find that.
I did tests with range cma_sport [0xC000, 0xFFFF] and cma_dport [1025,
0xFFFF] (but each test with one dport), and found:
1. The folding and non-folding results are similar;
2. When dport is multiple of 0x200 the result is very bad. I also tested
with your hashtest.c, there are much more "zero" hashes when sport or
dport is multiple of 0x200.
For the hash one of the original goal is symmetry, i.e.:
f(sport, dport) = f(dport, sport)
I'm very curious why this is a requirement. The hash is used to map
to a packet queue, which enforces ordering as well as providing a
congestion throttle point. These queues are one-way, and therefore
the same value has no effect when used symmetrically - it only works
one-way, the reverse flow is completely independent.
Am I missing something?
If that's not important I feel "sport * 31 + dport" [1] has a better result.
[1] https://www.strchr.com/hash_functions
Well, that'd be simple!
Tom.
