Preempt-RCU: update RCU Documentation.
This patch updates the RCU documentation to reflect preemptible RCU as well as recent publications. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Signed-off-by: Gautham R Shenoy <ego@in.ibm.com> Reviewed-by: Steven Rostedt <srostedt@redhat.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
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Paul E. McKenney
Ingo Molnar
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@ -9,8 +9,8 @@ The first thing resembling RCU was published in 1980, when Kung and Lehman
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[Kung80] recommended use of a garbage collector to defer destruction
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of nodes in a parallel binary search tree in order to simplify its
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implementation. This works well in environments that have garbage
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collectors, but current production garbage collectors incur significant
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read-side overhead.
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collectors, but most production garbage collectors incur significant
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overhead.
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In 1982, Manber and Ladner [Manber82,Manber84] recommended deferring
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destruction until all threads running at that time have terminated, again
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@ -99,16 +99,25 @@ locking, reduces contention, reduces memory latency for readers, and
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parallelizes pipeline stalls and memory latency for writers. However,
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these techniques still impose significant read-side overhead in the
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form of memory barriers. Researchers at Sun worked along similar lines
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in the same timeframe [HerlihyLM02,HerlihyLMS03]. These techniques
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can be thought of as inside-out reference counts, where the count is
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represented by the number of hazard pointers referencing a given data
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structure (rather than the more conventional counter field within the
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data structure itself).
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in the same timeframe [HerlihyLM02]. These techniques can be thought
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of as inside-out reference counts, where the count is represented by the
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number of hazard pointers referencing a given data structure (rather than
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the more conventional counter field within the data structure itself).
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By the same token, RCU can be thought of as a "bulk reference count",
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where some form of reference counter covers all reference by a given CPU
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or thread during a set timeframe. This timeframe is related to, but
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not necessarily exactly the same as, an RCU grace period. In classic
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RCU, the reference counter is the per-CPU bit in the "bitmask" field,
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and each such bit covers all references that might have been made by
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the corresponding CPU during the prior grace period. Of course, RCU
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can be thought of in other terms as well.
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In 2003, the K42 group described how RCU could be used to create
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hot-pluggable implementations of operating-system functions. Later that
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year saw a paper describing an RCU implementation of System V IPC
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[Arcangeli03], and an introduction to RCU in Linux Journal [McKenney03a].
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hot-pluggable implementations of operating-system functions [Appavoo03a].
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Later that year saw a paper describing an RCU implementation of System
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V IPC [Arcangeli03], and an introduction to RCU in Linux Journal
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[McKenney03a].
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2004 has seen a Linux-Journal article on use of RCU in dcache
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[McKenney04a], a performance comparison of locking to RCU on several
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@ -117,10 +126,19 @@ number of operating-system kernels [PaulEdwardMcKenneyPhD], a paper
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describing how to make RCU safe for soft-realtime applications [Sarma04c],
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and a paper describing SELinux performance with RCU [JamesMorris04b].
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2005 has seen further adaptation of RCU to realtime use, permitting
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2005 brought further adaptation of RCU to realtime use, permitting
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preemption of RCU realtime critical sections [PaulMcKenney05a,
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PaulMcKenney05b].
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2006 saw the first best-paper award for an RCU paper [ThomasEHart2006a],
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as well as further work on efficient implementations of preemptible
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RCU [PaulEMcKenney2006b], but priority-boosting of RCU read-side critical
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sections proved elusive. An RCU implementation permitting general
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blocking in read-side critical sections appeared [PaulEMcKenney2006c],
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Robert Olsson described an RCU-protected trie-hash combination
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[RobertOlsson2006a].
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Bibtex Entries
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@article{Kung80
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@ -203,6 +221,41 @@ Bibtex Entries
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,Address="New Orleans, LA"
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}
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@conference{Pu95a,
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Author = "Calton Pu and Tito Autrey and Andrew Black and Charles Consel and
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Crispin Cowan and Jon Inouye and Lakshmi Kethana and Jonathan Walpole and
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Ke Zhang",
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Title = "Optimistic Incremental Specialization: Streamlining a Commercial
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Operating System",
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Booktitle = "15\textsuperscript{th} ACM Symposium on
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Operating Systems Principles (SOSP'95)",
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address = "Copper Mountain, CO",
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month="December",
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year="1995",
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pages="314-321",
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annotation="
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Uses a replugger, but with a flag to signal when people are
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using the resource at hand. Only one reader at a time.
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"
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}
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@conference{Cowan96a,
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Author = "Crispin Cowan and Tito Autrey and Charles Krasic and
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Calton Pu and Jonathan Walpole",
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Title = "Fast Concurrent Dynamic Linking for an Adaptive Operating System",
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Booktitle = "International Conference on Configurable Distributed Systems
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(ICCDS'96)",
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address = "Annapolis, MD",
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month="May",
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year="1996",
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pages="108",
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isbn="0-8186-7395-8",
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annotation="
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Uses a replugger, but with a counter to signal when people are
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using the resource at hand. Allows multiple readers.
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"
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}
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@techreport{Slingwine95
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,author="John D. Slingwine and Paul E. McKenney"
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,title="Apparatus and Method for Achieving Reduced Overhead Mutual
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@ -312,6 +365,49 @@ Andrea Arcangeli and Andi Kleen and Orran Krieger and Rusty Russell"
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[Viewed June 23, 2004]"
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}
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@conference{Michael02a
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,author="Maged M. Michael"
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,title="Safe Memory Reclamation for Dynamic Lock-Free Objects Using Atomic
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Reads and Writes"
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,Year="2002"
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,Month="August"
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,booktitle="{Proceedings of the 21\textsuperscript{st} Annual ACM
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Symposium on Principles of Distributed Computing}"
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,pages="21-30"
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,annotation="
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Each thread keeps an array of pointers to items that it is
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currently referencing. Sort of an inside-out garbage collection
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mechanism, but one that requires the accessing code to explicitly
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state its needs. Also requires read-side memory barriers on
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most architectures.
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"
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}
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@conference{Michael02b
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,author="Maged M. Michael"
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,title="High Performance Dynamic Lock-Free Hash Tables and List-Based Sets"
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,Year="2002"
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,Month="August"
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,booktitle="{Proceedings of the 14\textsuperscript{th} Annual ACM
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Symposium on Parallel
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Algorithms and Architecture}"
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,pages="73-82"
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,annotation="
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Like the title says...
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"
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}
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@InProceedings{HerlihyLM02
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,author={Maurice Herlihy and Victor Luchangco and Mark Moir}
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,title="The Repeat Offender Problem: A Mechanism for Supporting Dynamic-Sized,
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Lock-Free Data Structures"
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,booktitle={Proceedings of 16\textsuperscript{th} International
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Symposium on Distributed Computing}
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,year=2002
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,month="October"
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,pages="339-353"
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}
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@article{Appavoo03a
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,author="J. Appavoo and K. Hui and C. A. N. Soules and R. W. Wisniewski and
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D. M. {Da Silva} and O. Krieger and M. A. Auslander and D. J. Edelsohn and
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@ -447,3 +543,95 @@ Oregon Health and Sciences University"
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Realtime turns into making RCU yet more realtime friendly.
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"
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}
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@conference{ThomasEHart2006a
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,Author="Thomas E. Hart and Paul E. McKenney and Angela Demke Brown"
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,Title="Making Lockless Synchronization Fast: Performance Implications
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of Memory Reclamation"
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,Booktitle="20\textsuperscript{th} {IEEE} International Parallel and
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Distributed Processing Symposium"
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,month="April"
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,year="2006"
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,day="25-29"
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,address="Rhodes, Greece"
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,annotation="
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Compares QSBR (AKA "classic RCU"), HPBR, EBR, and lock-free
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reference counting.
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"
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}
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@Conference{PaulEMcKenney2006b
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,Author="Paul E. McKenney and Dipankar Sarma and Ingo Molnar and
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Suparna Bhattacharya"
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,Title="Extending RCU for Realtime and Embedded Workloads"
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,Booktitle="{Ottawa Linux Symposium}"
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,Month="July"
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,Year="2006"
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,pages="v2 123-138"
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,note="Available:
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\url{http://www.linuxsymposium.org/2006/view_abstract.php?content_key=184}
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\url{http://www.rdrop.com/users/paulmck/RCU/OLSrtRCU.2006.08.11a.pdf}
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[Viewed January 1, 2007]"
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,annotation="
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Described how to improve the -rt implementation of realtime RCU.
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"
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}
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@unpublished{PaulEMcKenney2006c
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,Author="Paul E. McKenney"
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,Title="Sleepable {RCU}"
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,month="October"
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,day="9"
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,year="2006"
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,note="Available:
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\url{http://lwn.net/Articles/202847/}
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Revised:
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\url{http://www.rdrop.com/users/paulmck/RCU/srcu.2007.01.14a.pdf}
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[Viewed August 21, 2006]"
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,annotation="
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LWN article introducing SRCU.
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"
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}
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@unpublished{RobertOlsson2006a
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,Author="Robert Olsson and Stefan Nilsson"
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,Title="{TRASH}: A dynamic {LC}-trie and hash data structure"
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,month="August"
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,day="18"
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,year="2006"
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,note="Available:
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\url{http://www.nada.kth.se/~snilsson/public/papers/trash/trash.pdf}
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[Viewed February 24, 2007]"
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,annotation="
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RCU-protected dynamic trie-hash combination.
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"
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}
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@unpublished{ThomasEHart2007a
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,Author="Thomas E. Hart and Paul E. McKenney and Angela Demke Brown and Jonathan Walpole"
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,Title="Performance of memory reclamation for lockless synchronization"
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,journal="J. Parallel Distrib. Comput."
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,year="2007"
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,note="To appear in J. Parallel Distrib. Comput.
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\url{doi=10.1016/j.jpdc.2007.04.010}"
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,annotation={
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Compares QSBR (AKA "classic RCU"), HPBR, EBR, and lock-free
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reference counting. Journal version of ThomasEHart2006a.
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}
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}
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@unpublished{PaulEMcKenney2007QRCUspin
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,Author="Paul E. McKenney"
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,Title="Using Promela and Spin to verify parallel algorithms"
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,month="August"
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,day="1"
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,year="2007"
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,note="Available:
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\url{http://lwn.net/Articles/243851/}
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[Viewed September 8, 2007]"
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,annotation="
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LWN article describing Promela and spin, and also using Oleg
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Nesterov's QRCU as an example (with Paul McKenney's fastpath).
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"
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}
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@ -36,6 +36,14 @@ o How can the updater tell when a grace period has completed
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executed in user mode, or executed in the idle loop, we can
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safely free up that item.
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Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the
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same effect, but require that the readers manipulate CPU-local
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counters. These counters allow limited types of blocking
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within RCU read-side critical sections. SRCU also uses
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CPU-local counters, and permits general blocking within
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RCU read-side critical sections. These two variants of
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RCU detect grace periods by sampling these counters.
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o If I am running on a uniprocessor kernel, which can only do one
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thing at a time, why should I wait for a grace period?
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@ -46,7 +54,10 @@ o How can I see where RCU is currently used in the Linux kernel?
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Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
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"rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
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"srcu_read_lock", "srcu_read_unlock", "synchronize_rcu",
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"synchronize_net", and "synchronize_srcu".
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"synchronize_net", "synchronize_srcu", and the other RCU
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primitives. Or grab one of the cscope databases from:
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http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html
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o What guidelines should I follow when writing code that uses RCU?
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@ -67,7 +78,11 @@ o I hear that RCU is patented? What is with that?
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o I hear that RCU needs work in order to support realtime kernels?
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Yes, work in progress.
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This work is largely completed. Realtime-friendly RCU can be
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enabled via the CONFIG_PREEMPT_RCU kernel configuration parameter.
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However, work is in progress for enabling priority boosting of
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preempted RCU read-side critical sections.This is needed if you
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have CPU-bound realtime threads.
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o Where can I find more information on RCU?
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@ -46,12 +46,13 @@ stat_interval The number of seconds between output of torture
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shuffle_interval
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The number of seconds to keep the test threads affinitied
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to a particular subset of the CPUs. Used in conjunction
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with test_no_idle_hz.
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to a particular subset of the CPUs, defaults to 5 seconds.
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Used in conjunction with test_no_idle_hz.
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test_no_idle_hz Whether or not to test the ability of RCU to operate in
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a kernel that disables the scheduling-clock interrupt to
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idle CPUs. Boolean parameter, "1" to test, "0" otherwise.
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Defaults to omitting this test.
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torture_type The type of RCU to test: "rcu" for the rcu_read_lock() API,
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"rcu_sync" for rcu_read_lock() with synchronous reclamation,
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@ -82,8 +83,6 @@ be evident. ;-)
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The entries are as follows:
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o "ggp": The number of counter flips (or batches) since boot.
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o "rtc": The hexadecimal address of the structure currently visible
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to readers.
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@ -117,8 +116,8 @@ o "Reader Pipe": Histogram of "ages" of structures seen by readers.
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o "Reader Batch": Another histogram of "ages" of structures seen
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by readers, but in terms of counter flips (or batches) rather
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than in terms of grace periods. The legal number of non-zero
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entries is again two. The reason for this separate view is
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that it is easier to get the third entry to show up in the
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entries is again two. The reason for this separate view is that
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it is sometimes easier to get the third entry to show up in the
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"Reader Batch" list than in the "Reader Pipe" list.
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o "Free-Block Circulation": Shows the number of torture structures
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