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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>You can use the constant <tt>NCPU</tt> in kernel/param.h
<li>Let <tt>freerange</tt> give all free memory to the CPU
running <tt>freerange</tt>.
<li>The function <tt>cpuid</tt> returns the current core, but note
that you can use it when interrupts are turned off and so you will
need to turn on/off interrupts in your solution.
</ul>
<p>Run usertests to see if you don't break anything.
<h2>More scalabale bcache lookup</h2>
<p>Several processes reading different files repeatedly will
bottleneck in the buffer cache, bcache, in bio.c. Replace the
acquire in <tt>bget</tt> with
<pre>
while(!tryacquire(&bcache.lock)) {
printf("!");
}
</pre>
and run test0 from bcachetest and you will see "!"s.
<p>Modify <tt>bget</tt> so that a lookup for a buffer that is in the
bcache doesn't need to acquire <tt>bcache.lock</tt>. This is more
tricky than the kalloc assignment, because bcache buffers are truly
shared among processes. You must maintain the invariant that a
buffer is only once in memory.
<p> There are several races that <tt>bcache.lock</tt> protects
against, including:
<ul>
<li>A <tt>brelse</tt> may set <tt>b->ref</tt> to 0,
while concurrent <tt>bget</tt> is incrementing it.
<li>Two <tt>bget</tt> may see <tt>b->ref = 0</tt> and one may re-use
the buffer, while the other may replaces it with another block.
<li>A concurrent <tt>brelse</tt> modifies the list
that <tt>bget</tt> traverses.
</ul>
<p>A challenge is testing whether you code is still correct. One way
to do is to artificially delay certain operations
using <tt>sleepticks</tt>. <tt>test1</tt> trashes the buffer cache
and exercises more code paths.
<p>Here are some hints:
<ul>
<li>Read the description of buffer cache in the xv6 book (Section 7.2).
<li>Use a simple design: i.e., don't design a lock-free implementation.
<li>Use a simple hash table with locks per bucket.
<li>Searching in hash table for a buffer and allocating an entry
for that buffer when the buffer is not found must be atomic.
<li>It is fine to acquire <tt>bcache.lock</tt> in <tt>brelse</tt>
to update the LRU/MRU list.
</ul>
<p>Check that your implementation has less contention
on <tt>test0</tt>
<p>Make sure your implementation passes bcachetest and usertests.
<p>Optional:
<ul>
<li>make the buffer cache more scalable (e.g., avoid taking
out <tt>bcache.lock</tt> on <tt>brelse</tt>).
<li>make lookup lock-free (Hint: use gcc's <tt>__sync_*</tt>
functions.) How do you convince yourself that your implementation is correct?
</ul>
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