Re: KVM performance

[Anthony Liguori <aliguori-NZpS4cJIG2HvQtjrzfazuQ@public.gmane.org>]

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Rusty Russell wrote:
> On Sun, 2007-01-28 at 11:40 +0200, Avi Kivity wrote:
>   
>> Rusty Russell wrote:
>>  I use virtbench (http://ozlabs.org/~rusty/virtbench) to try
>>     
>>> to measure optimization results; it still needs more tests (and an
>>> explicit kvm backend).   
>>>       
>> A kvm backend would be appreciated.
>>     
>  
> Yes, and patches are most welcome 8)  Actually, I'll work on this week.

Attached patch was my initial attempt.  I wanted it to work with mostly 
unmodified guest images so the only requirement is that a getty is 
spawned on ttyS0.  I ran in to quite a few problems with virtbench 
though once I started launching multiple guests and haven't gotten 
around to debugging those yet.

Regards,

Anthony Liguori

>  
>   
>>>> - after modifying a pte, kvm doesn't preload the modified pte into 
>>>> shadow, but instead lets the guest fault it in  
>>>>         
>>> lguest doesn't either, but don't you still want the fault to update the
>>> host accessed bit?
>>>       
>> The story here is that the guest is handling a pagefault and writing the 
>> new guest pte.  kvm traps the write (guest pagetables are write 
>> protected), and has the option of updating the shadow pte to reflect the 
>> guest pte.
>>
>> The clever guest kernel will set the accessed bit (and the dirty bit on 
>> writable ptes) to avoid an rmw cycle by the hardware pagetable walker.
>>
>> [two instrumented runs later]
>>
>> Both Linux and Windows seem to do this optimization.
>>     
>  
> Right.  This (trivial!) optimization wins lguest a good 10%:
>  
> Before:
>     Time for one Copy-on-Write fault: 13622 nsec
>     Time to exec client once: 1085481 nsec
>     Time for one fork/exit/wait: 700796 nsec
> After:
>     Time for one Copy-on-Write fault: 12036 nsec
>     Time to exec client once: 969899 nsec
>     Time for one fork/exit/wait: 664601 nsec
>  
> Thanks!
> Rusty.
>  
>  
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[-- Attachment #2: virtbench-kvm.diff --]
[-- Type: text/x-patch, Size: 3265 bytes --]

diff -r 03813abef33e kvm/SETTINGS
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/kvm/SETTINGS	Mon Jan 15 15:50:09 2007 -0600
@@ -0,0 +1,3 @@
+# Sources from all scripts
+ROOT_FILE="/mnt/FC-5-i386.img"
+QEMU=qemu
diff -r 03813abef33e kvm/serial.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/kvm/serial.py	Mon Jan 15 16:00:13 2007 -0600
@@ -0,0 +1,58 @@
+import sys, socket, binascii, commands
+
+if len(sys.argv) != 9:
+    print 'Usage: serial IP PORT LOGIN PASSWORD PATH ID IP PORT'
+    sys.exit(1)
+
+sys.stderr.write('foo\n')
+
+login='login: '
+password='Password: '
+prompt=']# '
+
+s = socket.socket(socket.AF_INET)
+s.connect((sys.argv[1], int(sys.argv[2])))
+
+def wait_for(string):
+    buf = ''
+    while True:
+        f = s.recv(4096)
+        buf += f
+        if buf.endswith(string):
+            break
+
+s.sendall('\n')
+wait_for(login)
+s.sendall('%s\n' % sys.argv[3])
+
+wait_for(password)
+s.sendall('%s\n' % sys.argv[4])
+
+wait_for(prompt)
+status, output = commands.getstatusoutput('uuencode %s /tmp/virtclient' %
+                                          sys.argv[5])
+
+f = open(sys.argv[5])
+d = binascii.b2a_base64(f.read())
+f.close()
+
+s.sendall("stty -echo\n")
+wait_for(prompt)
+
+s.sendall("cat > /tmp/virtclient.b64 <<EOF\n")
+s.sendall(d)
+s.sendall("\nEOF\n")
+wait_for(prompt)
+
+s.sendall("""python
+import sys, binascii
+i = open('/tmp/virtclient.b64', 'r')
+f = open('/tmp/virtclient', 'w')
+f.write(binascii.a2b_base64(i.read()))
+sys.exit(0)
+""")
+wait_for(prompt)
+s.sendall('chmod 755 /tmp/virtclient\n')
+wait_for(prompt)
+s.sendall('/tmp/virtclient %s %s %s\n' % (sys.argv[6], sys.argv[7], sys.argv[8]))
+wait_for(prompt)
diff -r 03813abef33e kvm/start
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/kvm/start	Mon Jan 15 11:56:18 2007 -0600
@@ -0,0 +1,13 @@
+#! /bin/sh
+
+set -e
+
+. kvm/SETTINGS
+
+#if grep AuthenticAMD 2>/dev/null ; then
+#    modules="kvm kvm-amd"
+#else
+#    modules="kvm kvm-intel"
+#fi
+
+#[ -c /dev/kvm ] || modprobe $modules
diff -r 03813abef33e kvm/start_machine
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/kvm/start_machine	Sat Jan 27 11:09:40 2007 -0600
@@ -0,0 +1,10 @@
+#! /bin/sh
+
+. kvm/SETTINGS
+
+serial_port=$((1025 + $1))
+
+$QEMU -m 128 -hda "${ROOT_FILE}" -snapshot -serial tcp:localhost:${serial_port},server,nowait -kernel-kqemu &
+sleep 2
+python kvm/serial.py 127.0.0.1 ${serial_port} root ibm4xen virtclient $1 $2 $3 &
+jobs -p %1
diff -r 03813abef33e kvm/stop
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/kvm/stop	Mon Jan 15 11:32:55 2007 -0600
@@ -0,0 +1,4 @@
+#! /bin/sh
+
+. kvm/SETTINGS
+exit 0
diff -r 03813abef33e kvm/stop_machine
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/kvm/stop_machine	Mon Jan 15 11:43:29 2007 -0600
@@ -0,0 +1,5 @@
+#! /bin/sh
+
+. kvm/SETTINGS
+
+kill $1
diff -r 03813abef33e server.c
--- a/server.c	Thu Jan 11 13:56:30 2007 +1100
+++ b/server.c	Mon Jan 15 16:02:11 2007 -0600
@@ -227,7 +227,7 @@ static struct sockaddr_in get_server_add
 	socklen_t socklen = sizeof(saddr);
 
 	/* This assumes we have an eth0. */
-	strcpy(ifr.ifr_name, "eth0");
+	strcpy(ifr.ifr_name, "ath0");
 	sin->sin_family = AF_INET;
 	if (ioctl(sock, SIOCGIFADDR, &ifr) != 0)
 		err(1, "Getting interface address for eth0");

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