RCU is well-known at the kernel-level for providing a way to synchronize shared data structures in read-often, update-rarely scenarios. The development of a RCU library at the userspace application level has been mainly driven by the need for efficient synchronization of userspace tracing control data structures. IBM kindly agreed to allow distribution of RCU-related code in a LGPL library, which makes it available for everyone to use. This can have large impact on the design of highly scalable applications performing caching of frequent requests, like domain name servers, proxy and web servers. This presentation will discuss about the class of applications which could benefit from using the userspace RCU library. The userspace RCU library is available under the LGPL license at http://www.lttng.org/urcu .

1. Userspace RCU Library:
What Linear Multiprocessor
Scalability Means for Your
Application
Linux Plumbers Conference 2009
Mathieu Desnoyers
École Polytechnique de Montréal

2. > Mathieu Desnoyers
● Author/maintainer of :
– LTTV (Linux Trace Toolkit Viewer)
● 2003-...
– LTTng (Linux Trace Toolkit Next Generation)
● 2005-...
– Immediate Values
● 2007...
– Tracepoints
● 2008-...
– Userspace RCU Library
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● 2009-...

3. > Contributions by
● Paul E. McKenney
– IBM Linux Technology Center
● Alan Stern
– Rowland Institute, Harvard University
● Jonathan Walpole
– Computer Science Department, Portland
State University
● Michel Dagenais
– Computer and Software Engineering Dpt.,
3 École Polytechnique de Montréal

4. > Summary
● RCU Overview
● Kernel vs Userspace RCU
● Userspace RCU Library
● Benchmarks
● RCU-Friendly Applications
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5. > Linux Kernel RCU Usage
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6. > RCU Overview
● Relativistic programming
– Updates seen in different orders by CPUs
– Tolerates conflicts
● Linear scalability
● Wait-free read-side
● Efficient updates
– Only a single pointer exchange needs
exclusive access
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7. > Schematic of RCU Update
and Read-Side C.S.
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8. > RCU Linked-List Deletion
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9. > Kernel vs Userspace RCU
● Quiescent state
– Kernel threads
● Wait for kernel pre-existing RCU read-side C.S. to
complete
– User threads
● Wait for process pre-existing RCU read-side C.S. to
complete
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10. > Userspace RCU Library
● QSBR
– liburcu-qsbr.so
● Generic RCU
– liburcu-mb.so
● Signal-based RCU
– liburcu.so
● call_rcu()
– liburcu-defer.so
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11. > QSBR
● Detection of quiescent state:
– Each reader thread calls
rcu_quiescent_state() periodically.
● Require application modification
● Read-side with very low overhead
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12. > Generic RCU
● Detection of quiescent state:
– rcu_read_lock()/rcu_read_unlock() mark the
beginning/end of the critical sections
– Counts nesting level
● Suitable for library use
● Higher read-side overhead than QSBR
due to added memory barriers
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13. > Signal-based RCU
● Same quiescent state detection as
Generic RCU
● Suitable for library use, but reserves a
signal
● Read-side close to QSBR performance
– Remove memory barriers from
rcu_read_lock()/rcu_read_unlock().
– Replaced by memory barriers in signal
handler, executed at each update-side
memory barrier.
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14. > call_rcu()
● Eliminates the need to call
synchronize_rcu() after each removal
● Queues RCU callbacks for deferred
batched execution
● Wait-free unless per-thread queue is full
● “Worker thread” executes callbacks
periodically
● Energy-efficient, uses sys_futex()
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15. > Example: RCU Read-Side
struct mystruct *rcudata = &somedata;
/* register thread with rcu_register_thread()/rcu_unregister_thread() */
void fct(void)
{
struct mystruct *ptr;
rcu_read_lock();
ptr = rcu_dereference(rcudata);
/* use ptr */
rcu_read_unlock();
}
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16. > Example: exchange pointer
struct mystruct *rcudata = &somedata;
void replace_data(struct mystruct data)
{
struct mystruct *new, *old;
new = malloc(sizeof(*new));
memcpy(new, &data, sizeof(*new));
old = rcu_xchg_pointer(&rcudata, new);
call_rcu(free, old);
}
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17. > Example:
compare-and-exchange pointer
struct mystruct *rcudata = &somedata;
/* register thread with rcu_register_thread()/rcu_unregister_thread() */
void modify_data(int increment_a, int increment_b)
{
struct mystruct *new, *old;
new = malloc(sizeof(*new));
rcu_read_lock(); /* Ensure pointer is not re-used */
do {
old = rcu_dereference(rcudata);
memcpy(new, old, sizeof(*new));
new->field_a += increment_a;
new->field_b += increment_b;
} while (rcu_cmpxchg_pointer(&rcudata, old, new) != old);
rcu_read_unlock();
call_rcu(free, old);
}
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18. > Benchmarks
● Read-side Scalability
● Read-side C.S. length impact
● Update Overhead
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19. > Read-Side Scalability
64-cores POWER5+
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20. > Read-Side C.S. Length Impact
64-cores POWER5+, logarithmic scale (x, y)
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21. > Update Overhead
64-cores POWER5+, logarithmic scale (x, y)
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22. > RCU-Friendly Applications
● Multithreaded applications with read-
often shared data
– Cache
● Name servers
● Proxy
● Web servers with static pages
– Configuration
● Low synchronization overhead
● Dynamically modified without restart
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23. > RCU-Friendly Applications
● Libraries supporting multithreaded
applications
– Tracing library, e.g. lib UST (LTTng port for
userspace tracing)
● http://git.dorsal.polymtl.ca/?p=ust.git
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24. > RCU-Friendly Applications
● Libraries supporting multithreaded
applications (cont.)
– Typing/data structure support
● Typing system
– Creation of a class is a rare event
– Reading class structure happens at object
creation/destruction (_very_ often)
– Applies to gobject
● Used by: gtk/gdk/glib/gstreamer...
● Efficient hash tables
● Glib “quarks”
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25. > RCU-Friendly Applications
● Routing tables in userspace
● Userspace network stacks
● Userspace signal-handling
– Signal-safe read-side
– Could implement an inter-thread signal
multiplexer
● Your own ?
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26. > Info / Download / Contact
● Mathieu Desnoyers
– Computer and Software Engineering Dpt.,
École Polytechnique de Montréal
● Web site:
– http://www.lttng.org/urcu
● Git tree
– git://lttng.org/userspace-rcu.git
● Email
– mathieu.desnoyers@polymtl.ca
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