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futex.2: Add discussion of FUTEX_WAIT_BITSET/FUTEX_WAKE_BITSET "multiplexing" 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2015-01-13 16:29:01 +01:00
parent 98d769c098
commit e9d4496bf3
1 changed files with 39 additions and 1 deletions
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man2/futex.2
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@ -454,11 +454,49 @@ The selection is done by a bit-wise AND of the "wake" bitset
(i.e., the value in
.IR val3 )
and the bitset which is stored in the kernel-internal
state of the waiter (using
state of the waiter (the "wait" biset that is set using
.BR FUTEX_WAIT_BITSET ).
All of the waiters for which the result of the AND is nonzero are woken up;
the remaining waiters are left sleeping.
.\" FIXME please review this paragraph that I added
The effect of
.BR FUTEX_WAIT_BITSET
and
.BR FUTEX_WAKE_BITSET
is to allow selective wake-ups among multiple waiters that are waiting
on the same futex;
since a futex has a size of 32 bits,
these operations provide 32 wakeup "channels".
(The
.BR FUTEX_WAIT
and
.BR FUTEX_WAKE
operations correspond to
.BR FUTEX_WAIT_BITSET
and
.BR FUTEX_WAKE_BITSET
operations where the bitsets are all ones.)
Note, however, that using this bitset multiplexing feature on a
futex is less efficient than simply using multiple futexes,
because employing bitset multiplexing requires the kernel
to check all waiters on a futex,
including those that are not interested in being woken up
(i.e., they do not have the relevant bit set in their "wait" bitset).
.\" According to http://locklessinc.com/articles/futex_cheat_sheet/:
.\"
.\" "The original reason for the addition of these extensions
.\" was to improve the performance of pthread read-write locks
.\" in glibc. However, the pthreads library no longer uses the
.\" same locking algorithm, and these extensions are not used
.\" without the bitset parameter being all ones.
.\"
.\" The page goes on to note that the FUTEX_WAIT_BITSET operation
.\" is nevertheless used (with a bitset of all ones) in order to
.\" obtain the absolute timeout functionality that is useful
.\" for efficiently implementing Pthreads APIs (which use absolute
.\" timeouts); FUTEX_WAIT provides only relative timeouts.
The
.I uaddr2
and