Date: Thu, 23 Aug 2018 06:58:17 -0600 From: Ian Lepore <ian@freebsd.org> To: Sebastian Huber <sebastian.huber@embedded-brains.de>, Hans Petter Selasky <hps@selasky.org>, Eugene Grosbein <eugen@grosbein.net>, FreeBSD <freebsd-hackers@freebsd.org> Subject: Re: epoch(9) background information? Message-ID: <1535029097.27158.180.camel@freebsd.org> In-Reply-To: <68c80dd6-a811-06c1-27d9-25e99e9fd4ed@embedded-brains.de> 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> <68c80dd6-a811-06c1-27d9-25e99e9fd4ed@embedded-brains.de>
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On Thu, 2018-08-23 at 12:59 +0200, Sebastian Huber wrote: > On 23/08/18 12:27, 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. > Yes, RTEMS has a critical_enter() to disable pre-emption (you could also > disable interrupts as a brute force means). > > You still have the lock-free data structure inside the critical section. > Currently, this is only ck_queue, so not a real problem. However, in > case some more advanced lock-less algorithms would appear with a bit of > spinning here and there, then this would need further adoptions for the > uni-processor system. Not all targets support C11 atomics in hardware, > some need libatomic (a GCC thing). > Freebsd runs on many single-processor systems on all supported architectures and that isn't going to change. -- Ian
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