epoch(9) background information?

[Sebastian Huber]

On 22/08/18 08:34, Sebastian Huber wrote:
> On 21/08/18 15:38, Jacques Fourie wrote:
>>
>>
>> On Tue, Aug 21, 2018 at 8:33 AM, Sebastian Huber 
>> <sebastian.huber at embedded-brains.de 
>> <mailto:sebastian.huber at embedded-brains.de>> wrote:
>>
>>     Hello,
>>
>>     I update currently a port of the FreeBSD network stack, etc. to
>>     the real-time operating system RTEMS from the head version at
>>     2017-04-04 to the head version of today. I noticed that some
>>     read-write locks are replaced by a relatively new stuff called
>>     EPOCH(9). Is there some background information available for this?
>>     The man page is a bit vague and searching for something named
>>     epoch on the internet is not really great. For example, what is
>>     the motivation for this change? How is this related to
>>     read-copy-update (RCU)?
>>
>>     --     Sebastian Huber, embedded brains GmbH
>>
>>     Address : Dornierstr. 4, D-82178 Puchheim, Germany
>> <https://maps.google.com/?q=Dornierstr.+4,+D-82178+Puchheim,+Germany&entry=gmail&source=g>
>>     Phone   : +49 89 189 47 41-16
>>     Fax     : +49 89 189 47 41-09
>>     E-Mail  : sebastian.huber at embedded-brains.de
>>     <mailto:sebastian.huber at embedded-brains.de>
>>     PGP     : Public key available on request.
>>
>>     Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des 
>> EHUG.
>>
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>>
>> Additional information is available here : 
>> http://concurrencykit.org/presentations/ebr.pdf 
>> <http://concurrencykit.org/presentations/ebr.pdf>. The way I 
>> understand it is that it is mostly used in place of read locks to 
>> provide liveness guarantees without using atomics. Additional detail 
>> is available in the commit messages. As an example see r333813 for 
>> some performance data.
>>
>
> Thanks, for the reference. The "epoch reclamation" are good keywords 
> to find more information.
>
> What is the right mailing list to ask questions about the epoch 
> implementation of the FreeBSD kernel?
>
> To support this machinery in RTEMS is a bit difficult (in particular 
> EPOCH_LOCKED). Since RTEMS is supposed to be a real-time operating 
> system it supports only fixed-priority and job-level fixed priority 
> (EDF) schedulers. To allow some scaling to larger SMP systems it 
> supports clustered scheduling together with the mutual exclusion 
> locking protocols MrsP 
> (http://www-users.cs.york.ac.uk/~burns/MRSPpaper.pdf) and OMIP 
> (http://www.mpi-sws.org/~bbb/papers/pdf/ecrts13b.pdf). This makes the 
> thread pinning hard to implement (which is very easy to support in 
> FreeBSD). The locking protocols may temporarily move a thread which 
> owns a mutex to a foreign scheduler instance, e.g. a thread which 
> wants to obtain the mutex helps the owner to make progress if it was 
> pre-empted in its home scheduler instance. Due to a timeout of the 
> helper the owner may loose the right to execute in the foreign 
> scheduler instance. This would make it impossible to fulfil the 
> processor pinning constraint (e.g. the thread priority in the foreign 
> scheduler instance is undefined).
>
> It would save me a lot of trouble if I could assume that EPOCH_LOCKED 
> is an exotic feature which is unlikely to get used in FreeBSD.
>

Another question, is it a common use case to call epoch_enter_preempt() 
and epoch_exit_preempt() while owning a mutex?

-- 
Sebastian Huber, embedded brains GmbH

Address : Dornierstr. 4, D-82178 Puchheim, Germany
Phone   : +49 89 189 47 41-16
Fax     : +49 89 189 47 41-09
E-Mail  : sebastian.huber at embedded-brains.de
PGP     : Public key available on request.

Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des EHUG.