rtfp.txt

linux 内核源代码

Read the F-ing Papers!


This document describes RCU-related publications, and is followed by
the corresponding bibtex entries.  A number of the publications may
be found at http://www.rdrop.com/users/paulmck/RCU/.

The first thing resembling RCU was published in 1980, when Kung and Lehman
[Kung80] recommended use of a garbage collector to defer destruction
of nodes in a parallel binary search tree in order to simplify its
implementation.  This works well in environments that have garbage
collectors, but current production garbage collectors incur significant
read-side overhead.

In 1982, Manber and Ladner [Manber82,Manber84] recommended deferring
destruction until all threads running at that time have terminated, again
for a parallel binary search tree.  This approach works well in systems
with short-lived threads, such as the K42 research operating system.
However, Linux has long-lived tasks, so more is needed.

In 1986, Hennessy, Osisek, and Seigh [Hennessy89] introduced passive
serialization, which is an RCU-like mechanism that relies on the presence
of "quiescent states" in the VM/XA hypervisor that are guaranteed not
to be referencing the data structure.  However, this mechanism was not
optimized for modern computer systems, which is not surprising given
that these overheads were not so expensive in the mid-80s.  Nonetheless,
passive serialization appears to be the first deferred-destruction
mechanism to be used in production.  Furthermore, the relevant patent has
lapsed, so this approach may be used in non-GPL software, if desired.
(In contrast, use of RCU is permitted only in software licensed under
GPL.  Sorry!!!)

In 1990, Pugh [Pugh90] noted that explicitly tracking which threads
were reading a given data structure permitted deferred free to operate
in the presence of non-terminating threads.  However, this explicit
tracking imposes significant read-side overhead, which is undesirable
in read-mostly situations.  This algorithm does take pains to avoid
write-side contention and parallelize the other write-side overheads by
providing a fine-grained locking design, however, it would be interesting
to see how much of the performance advantage reported in 1990 remains
in 2004.

At about this same time, Adams [Adams91] described ``chaotic relaxation'',
where the normal barriers between successive iterations of convergent
numerical algorithms are relaxed, so that iteration $n$ might use
data from iteration $n-1$ or even $n-2$.  This introduces error,
which typically slows convergence and thus increases the number of
iterations required.  However, this increase is sometimes more than made
up for by a reduction in the number of expensive barrier operations,
which are otherwise required to synchronize the threads at the end
of each iteration.  Unfortunately, chaotic relaxation requires highly
structured data, such as the matrices used in scientific programs, and
is thus inapplicable to most data structures in operating-system kernels.

In 1993, Jacobson [Jacobson93] verbally described what is perhaps the
simplest deferred-free technique: simply waiting a fixed amount of time
before freeing blocks awaiting deferred free.  Jacobson did not describe
any write-side changes he might have made in this work using SGI's Irix
kernel.  Aju John published a similar technique in 1995 [AjuJohn95].
This works well if there is a well-defined upper bound on the length of
time that reading threads can hold references, as there might well be in
hard real-time systems.  However, if this time is exceeded, perhaps due
to preemption, excessive interrupts, or larger-than-anticipated load,
memory corruption can ensue, with no reasonable means of diagnosis.
Jacobson's technique is therefore inappropriate for use in production
operating-system kernels, except when such kernels can provide hard
real-time response guarantees for all operations.

Also in 1995, Pu et al. [Pu95a] applied a technique similar to that of Pugh's
read-side-tracking to permit replugging of algorithms within a commercial
Unix operating system.  However, this replugging permitted only a single
reader at a time.  The following year, this same group of researchers
extended their technique to allow for multiple readers [Cowan96a].
Their approach requires memory barriers (and thus pipeline stalls),
but reduces memory latency, contention, and locking overheads.

1995 also saw the first publication of DYNIX/ptx's RCU mechanism
[Slingwine95], which was optimized for modern CPU architectures,
and was successfully applied to a number of situations within the
DYNIX/ptx kernel.  The corresponding conference paper appeared in 1998
[McKenney98].

In 1999, the Tornado and K42 groups described their "generations"
mechanism, which quite similar to RCU [Gamsa99].  These operating systems
made pervasive use of RCU in place of "existence locks", which greatly
simplifies locking hierarchies.

2001 saw the first RCU presentation involving Linux [McKenney01a]
at OLS.  The resulting abundance of RCU patches was presented the
following year [McKenney02a], and use of RCU in dcache was first
described that same year [Linder02a].

Also in 2002, Michael [Michael02b,Michael02a] presented "hazard-pointer"
techniques that defer the destruction of data structures to simplify
non-blocking synchronization (wait-free synchronization, lock-free
synchronization, and obstruction-free synchronization are all examples of
non-blocking synchronization).  In particular, this technique eliminates
locking, reduces contention, reduces memory latency for readers, and
parallelizes pipeline stalls and memory latency for writers.  However,
these techniques still impose significant read-side overhead in the
form of memory barriers.  Researchers at Sun worked along similar lines
in the same timeframe [HerlihyLM02,HerlihyLMS03].  These techniques
can be thought of as inside-out reference counts, where the count is
represented by the number of hazard pointers referencing a given data
structure (rather than the more conventional counter field within the
data structure itself).

In 2003, the K42 group described how RCU could be used to create
hot-pluggable implementations of operating-system functions.  Later that
year saw a paper describing an RCU implementation of System V IPC
[Arcangeli03], and an introduction to RCU in Linux Journal [McKenney03a].

2004 has seen a Linux-Journal article on use of RCU in dcache
[McKenney04a], a performance comparison of locking to RCU on several
different CPUs [McKenney04b], a dissertation describing use of RCU in a
number of operating-system kernels [PaulEdwardMcKenneyPhD], a paper
describing how to make RCU safe for soft-realtime applications [Sarma04c],
and a paper describing SELinux performance with RCU [JamesMorris04b].

2005 has seen further adaptation of RCU to realtime use, permitting
preemption of RCU realtime critical sections [PaulMcKenney05a,
PaulMcKenney05b].

Bibtex Entries

@article{Kung80
,author="H. T. Kung and Q. Lehman"
,title="Concurrent Maintenance of Binary Search Trees"
,Year="1980"
,Month="September"
,journal="ACM Transactions on Database Systems"
,volume="5"
,number="3"
,pages="354-382"
}

@techreport{Manber82
,author="Udi Manber and Richard E. Ladner"
,title="Concurrency Control in a Dynamic Search Structure"
,institution="Department of Computer Science, University of Washington"
,address="Seattle, Washington"
,year="1982"
,number="82-01-01"
,month="January"
,pages="28"
}

@article{Manber84
,author="Udi Manber and Richard E. Ladner"
,title="Concurrency Control in a Dynamic Search Structure"
,Year="1984"
,Month="September"
,journal="ACM Transactions on Database Systems"
,volume="9"
,number="3"
,pages="439-455"
}

@techreport{Hennessy89
,author="James P. Hennessy and Damian L. Osisek and Joseph W. {Seigh II}"
,title="Passive Serialization in a Multitasking Environment"
,institution="US Patent and Trademark Office"
,address="Washington, DC"
,year="1989"
,number="US Patent 4,809,168 (lapsed)"
,month="February"
,pages="11"
}

@techreport{Pugh90
,author="William Pugh"
,title="Concurrent Maintenance of Skip Lists"
,institution="Institute of Advanced Computer Science Studies, Department of Computer Science, University of Maryland"
,address="College Park, Maryland"
,year="1990"
,number="CS-TR-2222.1"
,month="June"
}

@Book{Adams91
,Author="Gregory R. Adams"
,title="Concurrent Programming, Principles, and Practices"
,Publisher="Benjamin Cummins"
,Year="1991"
}

@unpublished{Jacobson93
,author="Van Jacobson"
,title="Avoid Read-Side Locking Via Delayed Free"
,year="1993"
,month="September"
,note="Verbal discussion"
}

@Conference{AjuJohn95
,Author="Aju John"
,Title="Dynamic vnodes -- Design and Implementation"
,Booktitle="{USENIX Winter 1995}"
,Publisher="USENIX Association"
,Month="January"
,Year="1995"
,pages="11-23"
,Address="New Orleans, LA"
}

@techreport{Slingwine95
,author="John D. Slingwine and Paul E. McKenney"
,title="Apparatus and Method for Achieving Reduced Overhead Mutual
Exclusion and Maintaining Coherency in a Multiprocessor System
Utilizing Execution History and Thread Monitoring"
,institution="US Patent and Trademark Office"
,address="Washington, DC"
,year="1995"
,number="US Patent 5,442,758 (contributed under GPL)"
,month="August"
}

@techreport{Slingwine97
,author="John D. Slingwine and Paul E. McKenney"
,title="Method for maintaining data coherency using thread
activity summaries in a multicomputer system"
,institution="US Patent and Trademark Office"
,address="Washington, DC"
,year="1997"
,number="US Patent 5,608,893 (contributed under GPL)"