On Sun, Jun 5, 2016 at 11:06 PM, Alexander V. Chernikov < melif...@freebsd.org> wrote:
> 06.06.2016, 04:40, "George Neville-Neil" <g...@freebsd.org>: > > On 4 Jun 2016, at 15:05, Alexander V. Chernikov wrote: > > > >> 02.06.2016, 20:51, "George V. Neville-Neil" <g...@freebsd.org>: > >>> Author: gnn > >>> Date: Thu Jun 2 17:51:29 2016 > >>> New Revision: 301217 > >>> URL: https://svnweb.freebsd.org/changeset/base/301217 > >>> > >>> Log: > >>> This change re-adds L2 caching for TCP and UDP, as originally > >>> added in D4306 > >>> but removed due to other changes in the system. Restore the > >>> llentry pointer > >>> to the "struct route", and use it to cache the L2 lookup (ARP or > >>> ND6) as > >>> appropriate. > >> > >> I have several comments regarding this commit. > >> > >> 1 Architecturally, there was quite a lot of efforts to eliminate > >> layering violation between lltable and other places in network stack. > >> It ended by committing D4102, which allowed both to cleanup lower > >> level, provide abstract “prepend” framework which could be used to > >> provide cached data to _otuput() functions. > >> This change brings these violations back in a really invasive way. > >> > >> Additionally, implementing L2 PCB caching at the other subsystem > >> expense is really a bad idea. > >> If one wants caching in some subsystem, it should be implemented in > >> that subsystem not polluting other things. > >> Current implementation permits this by filling in “ro_prepend” / > >> ro_plen fields. > >> > >> In general, this change looks more like a local hack and not like the > >> code that should be included in the tree. > >> > >> 2 There was no benchmarks proving the effectiveness of this change. > >> (For example, it is not obvious if it could significantly improve TCP > >> since we still have per-session TCP wlock + (typically) per-ring > >> mutex, so removing lltable rock might not help things here). Given > >> that the patch complicates existing code, there should be adequate > >> benefits to consider whether this change is worth implementing. > >> > >> 3 The “network” group was not included to the review despite the > >> fact that most of the changes were related to the L2 layer which is > >> not “transport”, so some people might have missed this review. > >> > >> 4 This change DOES NOT WORK. really. > >> (which raises questions on both review and benchmarking process). > >> > >> The reason is that “plle” argument is filled only in “heavy” > >> lltable lookup functions (e.g. when we don’t have neighbour > >> adjacency). 99.9% of the time arpresolve/nd6_resolve() returns the > >> result w/o calling their heavy versions, and the returned “plle” > >> is NULL. > >> > >> This can be easily verified by calling something like > >> dtrace -n 'fbt:kernel:ether_output:entry /arg3!=NULL&&((struct route > >> *)arg3)->ro_lle != NULL/ { stack(); }' > >> > >> Given that, I kindly ask you to backout this change. > > > > Hi, > > > > I'm going to limit the scope of this reply to just you, me and Mike > > Karels, from whom this originated. > No, please keep the discussion open. The decision on having that > particular L2 caching implementation (and L2 caching in general) is quite > important, so it would be great if all technical arguments were saved so > other people can (now or later) understand the decision details. > > Thanks for understanding. > > > > Best, > > George > > This commit does seem to undo the non-trivial layer separation efforts invested previously. The flow-table construction was meant to help accelerate TCP/UDP route lookups. The various aspects of the routing code took flow-table into consideration, and those code are still present even after this change. --Qing _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
