Date: Mon, 6 Jun 2016 09:32:08 -0700 From: "Qing" <tomelite82@gmail.com> To: <melifaro@freebsd.org>, <gnn@freebsd.org> Cc: <mike@karels.net>, <src-committers@freebsd.org>, <svn-src-all@freebsd.org>, <svn-src-head@freebsd.org> Subject: RE: svn commit: r301217 - in head/sys: net netinet netinet6 Message-ID: <000001d1c010$f8d603c0$ea820b40$@gmail.com> In-Reply-To: <3505271465193212@web29h.yandex.ru> References: <201606021751.u52HpTrH090384@repo.freebsd.org> <3448221465067132@web17h.yandex.ru> <B3E52539-01F4-480C-9196-F9EF197E4887@freebsd.org> <3505271465193212@web29h.yandex.ru>
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-----Original Message----- From: owner-src-committers@freebsd.org = [mailto:owner-src-committers@freebsd.org] On Behalf Of Alexander V. = Chernikov Sent: Sunday, June 05, 2016 11:07 PM To: George Neville-Neil <gnn@freebsd.org> Cc: Mike Karels <mike@karels.net>; src-committers = <src-committers@freebsd.org>; svn-src-all <svn-src-all@freebsd.org>; = svn-src-head <svn-src-head@freebsd.org> Subject: Re: svn commit: r301217 - in head/sys: net netinet netinet6 06.06.2016, 04:40, "George Neville-Neil" <gnn@freebsd.org>: > On 4 Jun 2016, at 15:05, Alexander V. Chernikov wrote: > >> 02.06.2016, 20:51, "George V. Neville-Neil" <gnn@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 =E2=80=9Cprepend=E2=80=9D 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 = =E2=80=9Cro_prepend=E2=80=9D / >> ro_plen fields. >> >> In general, this change looks more like a local hack and not like=20 >> 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=20 >> 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 =E2=80=9Cnetwork=E2=80=9D group was not included to the review = despite the >> fact that most of the changes were related to the L2 layer which is >> not =E2=80=9Ctransport=E2=80=9D, 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 =E2=80=9Cplle=E2=80=9D argument is filled only in = =E2=80=9Cheavy=E2=80=9D >> lltable lookup functions (e.g. when we don=E2=80=99t have neighbour >> adjacency). 99.9% of the time arpresolve/nd6_resolve() returns the >> result w/o calling their heavy versions, and the returned = =E2=80=9Cplle=E2=80=9D >> is NULL. >> >> This can be easily verified by calling something like >> dtrace -n 'fbt:kernel:ether_output:entry /arg3!=3DNULL&&((struct = route >> *)arg3)->ro_lle !=3D 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=20 > Karels, from whom this originated. >>No, please keep the discussion open. The decision on having that = particular L2 caching=20 >>implementation (and L2 caching in general) is quite important, so it = would be great if=20 >>all technical arguments were saved so other people can=20 >>(now or later) understand the decision details. >>Thanks for understanding. This commit does seem to undo the non-trivial layer separation efforts = invested previously.=20 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
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