Date: Thu, 23 Aug 2018 10:37:24 -0400 From: Mark Johnston <markj@freebsd.org> To: Hans Petter Selasky <hps@selasky.org> Cc: Sebastian Huber <sebastian.huber@embedded-brains.de>, Eugene Grosbein <eugen@grosbein.net>, FreeBSD <freebsd-hackers@freebsd.org> Subject: Re: epoch(9) background information? Message-ID: <20180823143724.GB68810@raichu> In-Reply-To: <90e16238-6e4d-5d3d-499d-2a19a49be78c@selasky.org> References: <db397431-2c4c-64de-634a-20f38ce6a60e@embedded-brains.de> <3bfedcc3-0dae-7979-2bd4-da83f2c67e87@embedded-brains.de> <5B7E7804.4030907@grosbein.net> <978ae736-89b9-6d83-e2a1-d2834ca8ae55@embedded-brains.de> <90e16238-6e4d-5d3d-499d-2a19a49be78c@selasky.org>
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On Thu, Aug 23, 2018 at 12:27:12PM +0200, Hans Petter Selasky wrote: > On 8/23/18 11:28 AM, Sebastian Huber wrote: > > On 23/08/18 11:01, Eugene Grosbein wrote: > >> On 23.08.2018 15:39, Sebastian Huber wrote: > >> > >>> We used the FreeBSD network stack also on low-end targets > >>> (uni-processor) such as MCF548x ColdFire, Atmel SAM V71, SPARC LEON, > >>> etc. in current production environments (not legacy systems). The > >>> introduction of lock-free data structures (Concurrency Kit) and this > >>> epoch memory reclamation makes little sense on these targets (at least > >>> from my point of view). However, FreeBSD has still the SMP configuration > >>> option (sys/conf/options) which suggests that SMP is optional. Is a > >>> uni-processor system something which is considered by the FreeBSD > >>> community as a thing worth supporting or can I expect that this is an > >>> exotic environment which will get less and less well supported in the > >>> future? I just need some guidance so that I can better plan for future > >>> FreeBSD baseline updates. > >> FreeBSD as virtualized uniprocessor guest should be supported at full > >> scale, > >> as well as embedded applications using single core x86 and non-x86 CPUs. > > > > If something should be supported, then there must be also someone who > > ensures that this is actually the case. I don't know the FreeBSD > > community good enough to judge if there is sufficient > > manpower/funding/interest for a well supported uni-processor FreeBSD. > > From the commits it is clear that FreeBSD receives a lot of attention > > from CDN providers such as Netflix and Limelight Networks. They probably > > don't care about uni-processor system support at all. The use of > > lock-free data structures (Concurrency Kit) and the epoch memory > > reclamation are now a mandatory infrastructure. There is no FreeBSD > > configuration option to avoid this. > > > > The Concurrency Kit in sys/contrib/ck has no explicit support for the > > FreeBSD RISC-V and MIPS architectures. So, I guess the fall-back > > sys/contrib/ck/include/gcc/ck_pr.h is used. The atomic support in > > sys/contrib/ck partially duplicates/extends the general atomic support > > of the FreeBSD kernel ATOMIC(9). To me it is a bit unclear what will be > > the future direction in the FreeBSD kernel with respect to lock-free > > data structures. > > > > Hi Sebastian, > > Do you have something like critical_enter() to disable pre-emption in > your OS? If you don't need to support SMP, the CPU pinning in the EPOCH > can be replaced by a critial_enter() / critial_exit() pair. Threads in preemptible epoch sections are allowed to acquire locks, so critical_enter()/exit() isn't a suitable replacement for epoch sections. I think a fallback implementation could be written without CK for the !SMP case, but it would require some work.
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