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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
Showing 165 out of 1137
Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 165
134
PROCESSES AND THREADS
CHAP. 2
The mutex system that we have described above is a bare-bones set of calls.
With all software, there is always a demand for more features, and synchronization
primitives are no exception. For example, sometimes a thread package offers a call
mutex
trylock
that either acquires the lock or returns a code for failure, but does
not block. This call gives the thread the flexibility to decide what to do next if there
are alternatives to just waiting.
There is a subtle issue that up until now we have glossed over but which is
worth at least making explicit. With a user-space threads package there is no prob-
lem with multiple threads having access to the same mutex, since all the threads
operate in a common address space. However, with most of the earlier solutions,
such as Peterson’s algorithm and semaphores, there is an unspoken assumption that
multiple processes have access to at least some shared memory, perhaps only one
word, but something.
If processes have disjoint address spaces, as we have consis-
tently said, how can they share the
turn
variable in Peterson’s algorithm, or sema-
phores or a common buffer?
There are two answers. First, some of the shared data structures, such as the
semaphores, can be stored in the kernel and accessed only by means of system
calls. This approach eliminates the problem.
Second, most modern operating sys-
tems (including UNIX and Windows) offer a way for processes to share some por-
tion of their address space with other processes.
In this way, buffers and other data
structures can be shared.
In the worst case, that nothing else is possible, a shared
file can be used.
If two or more processes share most or all of their address spaces, the dis-
tinction between processes and threads becomes somewhat blurred but is neverthe-
less present. Two processes that share a common address space still have different
open files, alarm timers, and other per-process properties, whereas the threads
within a single process share them. And it is always true that multiple processes
sharing a common address space never have the efficiency of user-level threads
since the kernel is deeply involved in their management.
Futexes
With increasing parallelism, efficient synchronization and locking is very im-
portant for performance.
Spin locks are fast if the wait is short, but waste CPU
cycles if not.
If there is much contention, it is therefore more efficient to block the
process and let the kernel unblock it only when the lock is free. Unfortunately, this
has the inverse problem: it works well under heavy contention, but continuously
switching to the kernel is expensive if there is very little contention to begin with.
To make matters worse, it may not be easy to predict the amount of lock con-
tention.
One interesting solution that tries to combine the best of both worlds is known
as
futex
, or ‘‘fast user space mutex.’’ A futex is a feature of Linux that implements
basic locking (much like a mutex) but avoids dropping into the kernel unless it
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