Dear subscribers,I have recently enabled and turned on Receive Side Scaling/Steering. The main reason was to use different hashing for the distribution of UDP datagrams over multiple paths. RSS uses 2-tuple hashing for UDP, thus UDP transmission from one host to another takes always the same path. Without RSS bits enabled hashes computed for choosing the path in MULTIPATH are done for both UDP and TCP on the same basis of srcip:port dstip:port. At a glance "options RSS" seems to be a hidden feature. There is no RSS(9), though PCBGROUP(9) refers to that page, neither RSS is present in LINT. Anyway, it can be added to the custom kernel and turned on. From time to time I am profiling the system with pmcstat(8). It seems that when IPv6 traffic is processed, the computations of the Toeplitz hashes (with RSS enabled) take a higher share of CPU cycles, which is expected, but the overall share of CPU cycles spent on the computations of these hashes is unexpectedly high (when compared for example with computation of different hashes, fib lookups etc.) and seems to be the cost (drawback ?) of enabling RSS bytes. I can share the samples and flame graphs. If anyone is concerned, please PM me. Digging a bit into the code I found out that our implementation is relatively simple[1] when compared with for example approach taken by DragonFlyBSD[2]. On the other hand, there is a notion of the importance of caching in our code[3]. I am not the programmer, perhaps refactoring won't help much in this case. Concluding, recently there is more cleanup of RSS code in the source repository than any progress or refactoring. Please let me ask then if these RSS bytes are still relevant and worth enabling or should be considered abandoned and going to be purged with time.
[1] https://cgit.freebsd.org/src/tree/sys/net/toeplitz.c[2] https://github.com/DragonFlyBSD/DragonFlyBSD/blob/master/sys/net/toeplitz.c
[3] https://cgit.freebsd.org/src/tree/sys/net/toeplitz.h Yours sincerely -- Marek Zarychta
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