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