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) If that's not important I feel "sport * 31 + dport" [1] has a better result. [1] https://www.strchr.com/hash_functions > > Tom.
