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74 lines
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<title>Implementation (Linux x86) - SBCL Internals</title>
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<a name="Implementation-(Linux-x86)"></a>
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Up: <a rel="up" accesskey="u" href="Threads.html#Threads">Threads</a>
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<h3 class="section">10.1 Implementation (Linux x86/x86-64)</h3>
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<p>Threading is implemented using pthreads and some Linux specific bits
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like futexes.
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<p>On x86 the per-thread local bindings for special variables is achieved
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using the %fs segment register to point to a per-thread storage area.
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This may cause interesting results if you link to foreign code that
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expects threading or creates new threads, and the thread library in
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question uses %fs in an incompatible way. On x86-64 the r12 register
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has a similar role.
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<p>Queues require the <code>sys_futex</code> system call to be available: this
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is the reason for the NPTL requirement. We test at runtime that this
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system call exists.
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<p>Garbage collection is done with the existing Conservative Generational
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GC. Allocation is done in small (typically 8k) regions: each thread
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has its own region so this involves no stopping. However, when a
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region fills, a lock must be obtained while another is allocated, and
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when a collection is required, all processes are stopped. This is
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achieved by sending them signals, which may make for interesting
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behaviour if they are interrupted in system calls. The streams
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interface is believed to handle the required system call restarting
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correctly, but this may be a consideration when making other blocking
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calls e.g. from foreign library code.
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<p>Large amounts of the SBCL library have not been inspected for
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thread-safety. Some of the obviously unsafe areas have large locks
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around them, so compilation and fasl loading, for example, cannot be
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parallelized. Work is ongoing in this area.
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