epoch(9) background information?

[Hans Petter Selasky]

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.

--HPS