[ltt-dev] [rp] [RFC] URCU concurrent data structure API

[Mathieu Desnoyers]

* Mathieu Desnoyers (mathieu.desnoyers at efficios.com) wrote:
> * Paul E. McKenney (paulmck at linux.vnet.ibm.com) wrote:
> > On Wed, Aug 17, 2011 at 10:23:33PM -0400, Mathieu Desnoyers wrote:
> > > * Paul E. McKenney (paulmck at linux.vnet.ibm.com) wrote:
> > > > On Wed, Aug 17, 2011 at 12:40:39PM -0400, Mathieu Desnoyers wrote:
[...]
> > 
> > Actually, if you do use synchronize_rcu() anywhere in the library,
> > you need to explicitly prohibit the caller being in an RCU read-side
> > critical section.  So I am not so sure that doing synchronize_rcu()
> > in any library function is all that great an idea in the first
> > place, at least in the general case.
> > 
> > A library function doing synchronize_rcu() needs to be documented
> > and carefully handled to the same extent as would a library function
> > that does a blocking read across the network, right?
> 
> Definitely agreed. synchronize_rcu should really be frowned upon in
> data container libraries. I was mainly pointing out that having access
> directly to the library functions provides more freedom to the data
> structure implementers.
[...]
> 
> > 
> > > So far, doing macro magic with solution #3 appears to be the best
> > > approach we have. The current implementation (in urcu master branch
> > > HEAD) supports linking various flavors of the CDS library within the
> > > same application (using different symbols for each one).
> > > 
> > > Thoughts ?
> > 
> > I am not yet convinced that we want to abandon #1 so quickly.  There
> > will probably be yet more user-level RCU algorithms produced, both
> > within this library and in other libraries, so minimizing the code
> > where the compiler needs to know which user-level algorithm is in
> > use seems to me to be a very good thing.
> 
> Yes, agreed.
> 
> > For example, if someone implements their own user-level RCU, approach
> > #1 would allow them to use that in conjunction with the cds_ algorithms
> > without even recompiling the urcu library.  In contrast, approach #3
> > would require them to add their implementation to the urcu library,
> > which would be a bad thing if their RCU implementation was so specialized
> > that we didn't want to accept it into urcu, right?
> 
> One way to do that might be to provide an automated build system that
> generates .so for all urcu flavors from any given data container. E.g.,
> if we can automatically append suffixes to cds symbols, we could get
> somewhere. It would be like the map header trick, but automated.
> 
> > 
> > > Thanks for your feedback!
> > 
> > Definitely an interesting topic.  ;-)
> 
> I think there is an important point to discuss with respect to this
> question too: do we want to give the cds_ functions direct control on
> locking, or do we want to leave that to the caller ?
> 
> I can list at least one case where giving control over locking to the
> cds_ functions would be useful: if we want the update-side of RCU RB
> tree to eventually scale, we could try assigning sub-parts of the tree
> to different mutexes, and use one mutex for the first levels of the
> tree. This would require the data structure to tie the mutexes to the
> actual data, so there is really no way the application could do that.
> 
> So if we choose to let data structures control locking, then the
> decision to generate one cds .so per RCU flavor would start to make more
> sense, because RCU can be seen as "locking" in some ways (although it is
> more formally synchronization than locking).
> 
> Giving control over RCU read-side is currently not much of an issue,
> because we can afford to let the application keep the rcu read-side lock
> without much practical limitation for the current data structures we
> have, but I think we might get into limitations for other data
> structures in the future, as shown with my multi-lock tree example.

I thought about it some more, and had discussions with various people,
and there are a few reasons to go for a scheme where rcu read lock
should be taken by the caller, and to pass call_rcu as a parameter to
the data structure init function:

A) The advantage, as you pointed out, is that one single .so is enough
   to support all RCU flavors. Very convenient for external data
   structure containers.

B) It clearly documents where rcu read-side locks are needed, so the user
   keep control and in-depth understanding of their read-side locks.

C) When multiple API functions that require RCU read-side lock to be
   held (sometimes even the same lock) throughout a sequence of API
   calls, we have no choice but to let the caller hold the read-side
   lock.

D) Due to support of multiple nesting of rcu read-side lock, any
   "improvement" we could get by releasing the read-side lock in
   retry loops would vanish in the cases where we are called within
   nested C.S..

E) If a library uses synchronize_rcu, this should be clearly documented,
   and even frowned upon, because this involves important limitations on
   the design of the caller, and important performance hit. There are
   usually ways to reach the same result through use of call_rcu, which
   should really be used thoroughout these libraries.

F) It clearly documents when a data structure needs to use call_rcu
   internally.

G) Some very early benchmark results show that there is indeed not
   much performance gain to achieve by inlining call_rcu, even if it is
   a version with a cache for the "call_rcu structure" lookup
   (per-cpu/per-thread/global). So passing it as a parameter to
   the data structure init function should be fine, even in cases
   where it is called very often.

H) For use-cases where applications would like to use more than one
   RCU flavor concurrently (which is now supported), leaving management
   of RCU read-side C.S. to the reader allows the application to take
   more than one RCU read-side lock across API calls. It also lets the
   application specify its own call_rcu function that could handle more
   than one RCU flavor.

So for all these reasons, I will go back to the API we have in our last
release (0.6.4), therefore reverting some of the API changes I did on
the git urcu master branch.

Thank you everyone for this very precious feedback!

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com