From d82f43e0c747347de18737f4a446474207fb65b9 Mon Sep 17 00:00:00 2001
From: Frans Kaashoek <kaashoek@mit.edu>
Date: Sat, 27 Jul 2019 16:00:12 -0400
Subject: Checkpoint start of locking lab
---
labs/lock.html | 81 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
labs/syscall.html | 6 ++++-
2 files changed, 86 insertions(+), 1 deletion(-)
create mode 100644 labs/lock.html
(limited to 'labs')
diff --git a/labs/lock.html b/labs/lock.html
new file mode 100644
index 0000000..1278b53
--- /dev/null
+++ b/labs/lock.html
@@ -0,0 +1,81 @@
+<html>
+<head>
+<title>Lab: locks</title>
+<link rel="stylesheet" href="homework.css" type="text/css" />
+</head>
+<body>
+
+<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.
+
+</body>
+</html>
diff --git a/labs/syscall.html b/labs/syscall.html
index 1e6e504..68abad2 100644
--- a/labs/syscall.html
+++ b/labs/syscall.html
@@ -121,7 +121,7 @@ interrupts.
<p>
You should put the following example program in <tt>user/alarmtest.c</tt>:
-<b>XXX Insert the final program here</b>
+<b>XXX Insert the final program here; maybe just give the code in the repo</b>
<pre>
#include "kernel/param.h"
#include "kernel/types.h"
@@ -315,7 +315,11 @@ use only one CPU, which you can do by running
<p>Once you pass <tt>test0</tt> and <tt>test1</tt>, run usertests to
make sure you didn't break any other parts of the kernel.
+
+</body>
+</html>
+
--
cgit v1.2.3
From bdcd23ea49885b5fd205d2856073d3d89b3a9bc2 Mon Sep 17 00:00:00 2001
From: Frans Kaashoek <kaashoek@mit.edu>
Date: Sat, 27 Jul 2019 20:18:40 -0400
Subject: a bit more
---
labs/lock.html | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
(limited to 'labs')
diff --git a/labs/lock.html b/labs/lock.html
index 1278b53..5eddc58 100644
--- a/labs/lock.html
+++ b/labs/lock.html
@@ -72,7 +72,12 @@ workloads.
<p>Some hints:
<ul>
- <li>Initially divide the free memory equally among the different CPUs.
+ <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.
--
cgit v1.2.3