82 lines
2.8 KiB
HTML
82 lines
2.8 KiB
HTML
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<html>
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<head>
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<title>Lab: locks</title>
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<link rel="stylesheet" href="homework.css" type="text/css" />
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</head>
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<body>
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<h1>Lab: locks</h1>
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<p>In this lab you will try to avoid lock contention for certain
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workloads.
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<h2>lock contention</h2>
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<p>The program user/kalloctest stresses xv6's memory allocator: three
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processes grow and shrink there address space, which will results in
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many calls to <tt>kalloc</tt> and <tt>kfree</tt>,
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respectively. <tt>kalloc</tt> and <tt>kfree</tt>
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obtain <tt>kmem.lock</tt>. To see if there is lock contention for
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<tt>kmem.lock</tt> replace the call to <tt>acquire</tt>
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in <tt>kalloc</tt> with the following code:
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<pre>
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while(!tryacquire(&kmem.lock)) {
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printf("!");
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}
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</pre>
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<p><tt>tryacquire</tt> tries to acquire <tt>kmem.lock</tt>: if the
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lock is taking it returns false (0); otherwise, it returns true (1)
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and with the lock acquired. Your first job is to
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implement <tt>tryacquire</tt> in kernel/spinlock.c.
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<p>A few hints:
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<ul>
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<li>look at <tt>acquire</tt>.
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<li>don't forget to restore interrupts when acquision fails
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<li>Add tryacquire's signature to defs.h.
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</ul>
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<p>Run usertests to see if you didn't break anything. Note that
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usertests never prints "!"; there is never contention
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for <tt>kmem.lock</tt>. The caller is always able to immediately
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acquire the lock and never has to wait because some other process
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has the lock.
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<p>Now run kalloctest. You should see quite a number of "!" on the
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console. kalloctest causes many processes to contend on
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the <tt>kmem.lock</tt>. This lock contention is a bit artificial,
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because qemu is simulating 3 processors, but it is likely on real
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hardware, there would be contention too.
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<h2>Removing lock contention</h2>
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<p>The root cause of lock contention in kalloctest is that there is a
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single free list, protected by a single lock. To remove lock
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contention, you will have to redesign the memory allocator to avoid
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a single lock and list. The basic idea is to maintain a free list
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per CPU, each list with its own lock. Allocations and frees on each
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CPU can run in parallel, because each CPU will operate on a
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different list.
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<p> The main challenge will be to deal with the case that one CPU runs
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out of memory, but another CPU has still free memory; in that case,
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the one CPU must "steal" part of the other CPU's free list.
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Stealing may introduce lock contention, but that may be acceptable
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because it may happen infrequently.
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<p>Your job is to implement per-CPU freelists and stealing when one
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CPU is out of memory. Run kalloctest() to see if your
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implementation has removed lock contention.
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<p>Some hints:
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<ul>
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<li>Initially divide the free memory equally among the different CPUs.
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</ul>
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<p>Run usertests to see if you don't break anything.
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</body>
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</html>
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