Memory in vm half as fast after migration of vm.

olivierlambert

xe vm-param-list uuid=<VM UUID>. Compare the output before and after migration.

Andreas

@olivierlambert
Okay find a value that is different
the test is done on the new VM which has 6 GIB in Ram
This is the value before migration
memory-target ( RO): 0
after migration
memory-target ( RO): 6442450944
and after reboot of VM
memory-target ( RO): 0

olivierlambert

That's weird, despite the fact your VM is set to static, right? (same dynamic min, max and static max?)

edit: is it the only diff?

Andreas

@olivierlambert
no this change from this
memory-actual ( RO): 6442455040
to this
memory-actual ( RO): 6442450944

and its below but they probably have no significance
start-time
console-uuids
dom-id
VCPUs-utilisation
guest-metrics-last-updated

olivierlambert

That's weird. We'll see if we can reproduce this. @Darkbeldin will try when he can (probably in January)

Andreas

@olivierlambert
Okay thanks
and happy New Year

olivierlambert

You too!

Andreas

@olivierlambert
Sorry to disturb you.
Okay just to verify that there was nothing wrong with the physical servers.
So I took 2 identical PCs and installed clean new xcp-ng 8.2
then install a virtual machine with static 4GB of memory and with guest tools.
Install redis and ran the test
then migrated VM to other pc and ran the test and the speed was half.
Took out the result before and after attached the files.

1.Before migration.txt
2.After migration.txt

Darkbeldin

@andreas Hi Andreas,

After testing it on my side i can confirm i reproduce the issue.
I will discuss it at dev level and get back to you.

Andreas

@darkbeldin
Okay Thanks

Forza

@darkbeldin said in Memory in vm half as fast after migration of vm.:

@andreas Hi Andreas,

After testing it on my side i can confirm i reproduce the issue.
I will discuss it at dev level and get back to you.

This seems quite an important find. Please let is know how this goes.

Darkbeldin

@andreas

So I was doing some testing before reporting to dev team and I have a behavior I will like you to check if you reproduce:
my clean VM report like this

yachy@ubuntuyachy:~$ redis-benchmark -r 1000000 -n 2000000 -t get,set,lpush,lpop -P 16 -q
SET: 156152.41 requests per second
GET: 168180.28 requests per second
LPUSH: 156421.08 requests per second
LPOP: 159757.17 requests per second

That's my reference, when I migrate to another host it report like this:

yachy@ubuntuyachy:~$ redis-benchmark -r 1000000 -n 2000000 -t get,set,lpush,lpop -P 16 -q
SET: 55718.07 requests per second
GET: 58683.72 requests per second
LPUSH: 55742.91 requests per second
LPOP: 54775.01 requests per second

If I reboot it goes back to original reporting but if I migrate back to the original host without rebooting it report like that.

redis-benchmark -r 1000000 -n 2000000 -t get,set,lpush,lpop -P 16 -q
SET: 138092.94 requests per second
GET: 153151.08 requests per second
LPUSH: 147004.78 requests per second
LPOP: 148115.23 requests per second

So not perfect as reference but way better than after migration.
As I want to be thorough before reporting could you check if you reproduce that?
So:

migrate to another host
make the test
migrate back to the original host
make the test

Thanks for your help.

Andreas

@darkbeldin
Hello
I installed clean new xcp-ng 8.2 on 2 identical PCs name host1 and host2 then updated to latest "yum update"
then install a virtual machine ubuntu 20.04 with static 4GB of memory and with guest tools.
Install redis-server
Then I did the test
on host1
root@ramtest:/home/andreas# redis-benchmark -r 1000000 -n 2000000 -t get,set,lpush,lpop -P 16 -q
SET: 243368.20 requests per second
GET: 261917.23 requests per second
LPUSH: 257499.67 requests per second
LPOP: 264830.50 requests per second

Then migrate to host2 got lower speed
root@ramtest:/home/andreas# redis-benchmark -r 1000000 -n 2000000 -t get,set,lpush,lpop -P 16 -q
SET: 92055.60 requests per second
GET: 95297.09 requests per second
LPUSH: 95570.31 requests per second
LPOP: 95401.64 requests per second

Then back to host1 got almost the same speed
root@ramtest:/home/andreas# redis-benchmark -r 1000000 -n 2000000 -t get,set,lpush,lpop -P 16 -q
SET: 238010.23 requests per second
GET: 253100.48 requests per second
LPUSH: 259100.92 requests per second
LPOP: 259134.50 requests per second

Darkbeldin

@andreas Ok so migrating back to the original host give us a small perf issue but clearly not what we see when we migrate to another host.
I will report it like that thanks for the test Andreas

Andreas

@darkbeldin
Okay
Did more tests
Started on host1 normal speed
migrate to host2
make the test got lower speed
restart vm
make the test on host2
Got normal speed
migrate to host1
make the test got lower speed
migrate to host2 normal speed

so it seems to be something that happens after first migrating to another host

I have a third exactly the same pc i should test install on it
and see what happens if i move vm to host3 after moving to host2
but I have to do it tomorrow, I do not have time now.

Darkbeldin

@andreas Yes i tested it no need to do it, migrating to a third hosts result to half perf has first migration.

Darkbeldin

@andreas Ok so after discussing it with Dev team the issue has been identified.
The trouble is linked to TSC management in the VM.
You can work around the issue by setting the VM:

xe vm-param-set uuid=<VM_UUID> platform:tsc_mode=2

But be aware we can not recommend this settings to go to a production VM.
TSC clock won't be emulated at all if you enable this settings. So you might have some weird time behavior during migration.

Andreas

@darkbeldin Okay thanks
I did test this and it worked.

TheNorthernLight

@darkbeldin Hello, So for those of us in production, does this problem affect rolling pool upgrades?

If so, how do we fix this and update our pools without needing to explicitly shutdown VMs in the pool?

Andrew

@darkbeldin Is memory access actually slower or is a timing issue with the statistics?