[Engine-devel] Weird behavior of multiple SetVmTicket query

Alexander Wels awels at redhat.com
Thu Nov 21 13:28:32 UTC 2013 

On Thursday, November 21, 2013 07:18:42 AM Daniel Erez wrote:
> ----- Original Message -----
> 
> > From: "Einav Cohen" <ecohen at redhat.com>
> > To: "Livnat Peer" <lpeer at redhat.com>, "Eli Mesika" <emesika at redhat.com>,
> > "Omer Frenkel" <ofrenkel at redhat.com>, "Doron Fediuck"
> > <dfediuck at redhat.com>, "Oved Ourfalli" <ovedo at redhat.com> Cc:
> > "engine-devel" <engine-devel at ovirt.org>
> > Sent: Monday, November 18, 2013 11:19:23 PM
> > Subject: Re: [Engine-devel] Weird behavior of multiple SetVmTicket query
> > 
> > engine-core maintainers - this one is mainly for you - see below.
> > the most important question (I think) is: Is there any reason
> > to not introduced a "sync" flavor of RunMultipleActions (i.e.
> > one that will return from the backend only when all actions have
> > been completed, similarly to RunAction)?
> 
> There's a couple of years old patch exactly for that [1].
> It should allow both synchronous and asynchronous multiple actions according
> to a flag. Question is when do we want to use the synchronous flavor?
> The reasoning behind the current implementation is to return a quick
> response to the client, as executing multiple vdsm commands can potentially
> require a long wait. Do we need to introduce a sync version just for VM
> console connection flow? IIRC, the flow actually uses multiple calls to the
> singular form (RunAction) rather than RunMultipleAction?
> 
> [1] http://gerrit.ovirt.org/#/c/3210/
> 

We recently introduced a patch [1] that on the front-end is smart enough to 
consolidate multiple query/actions into a single runMultiple(Query/Action) 
call to reduce the number http requests to the back-end. In the SetVMTicket 
case it combined two actions into one runMultipleAction call and was expected 
that to return once all the actions where complete, which didn't happen 
causing some undesired behavior on the client side. This sparked the 
investigation that ended in this email.

So yes I would definitely like a 'synchronous' version of runMultipleAction so 
the frontend code can set that flag when combining multiple actions into a 
single call. We can only safely do this if we are combining several runActions 
as all of those are 'synchronous'.

[1] http://gerrit.ovirt.org/#/c/17356/

> > ----
> > Thanks,
> > Einav
> > 
> > ----- Original Message -----
> > 
> > > From: "Vojtech Szocs" <vszocs at redhat.com>
> > > To: awels at redhat.com
> > > Cc: "engine-devel" <engine-devel at ovirt.org>
> > > Sent: Tuesday, November 12, 2013 6:40:18 AM
> > > Subject: Re: [Engine-devel] Weird behavior of multiple SetVmTicket query
> > > 
> > > Forwarding this email to engine-devel so that backend maintainers are
> > > aware
> > > of this issue.
> > > 
> > > Looking at the code:
> > > 
> > > - MultipleActionsRunner#Execute first creates and "validates" all 
commands:
> > >   A. the "for" block which iterates over getParameters()
> > >   
> > >      1-> validate correlation ID, if OK create and add command,
> > >      otherwise
> > >      add
> > >      returnValue
> > >   
> > >   B the "if" block which tests getCommands().size()
> > >   
> > >      1-> if single command to execute, add its "canDoActionOnly"
> > >      returnValue,
> > >      which is returnValue with canDoAction but without actual result
> > >      object
> > >      2-> if multi commands to execute, execute chunks of max. 10 threads
> > >      (sequentially, ThreadPoolUtil.invokeAll returns after all threads
> > >      complete), same returnValue as above
> > >   
> > >   C. the "if" block which tests canRunActions
> > >   
> > >      1-> all commands are executed within SINGLE THREAD due to
> > >      ThreadPoolUtil.execute(Runnable), which is kind of weird comapred
> > >      to
> > >      how returnValues are prepared (see B2)
> > >      2-> when executing command, code DOES NOT CARE about its
> > >      returnValue,
> > >      i.e. returnValue was already prepared (see B) and command execution
> > >      should just update it
> > > 
> > > The problem (I think) is that C1 starts a different thread (to execute
> > > all
> > > commands) and immediately returns, i.e. code doesn't wait for thread to
> > > complete. This is why returnValues are observed on frontend as
> > > inconsistent.
> > > 
> > > Additionally, we're also mixing of two different things: canDoAction
> > > processing and returnValues processing. IMHO this should be refactored
> > > to
> > > make code easier to read.
> > > 
> > > Changes done by Alex (patch attached):
> > > 
> > > X1. returnValues changed to Collections.synchronizedList
> > > 
> > >     -> this means all access to returnValues is now serial
> > >     -> iteration over synchronizedList should also be enclosed in
> > >     
> > >     "synchronized (list)" block, so this:
> > >            for (VdcReturnValueBase value : returnValues) ...
> > >        
> > >        should be this:
> > >            synchronized (returnValues) { for (VdcReturnValueBase value :
> > >            returnValues) ... }
> > > 
> > > X2. commented-out original command execution via
> > > ThreadPoolUtil.execute(Runnable)
> > > 
> > >     -> new RunCommands method invokes all commands each in separate
> > >     thread
> > >     via ThreadPoolUtil.invokeAll
> > >     -> returnValues list is explicitly updated
> > > 
> > > Guys, what do you think?
> > > 
> > > Vojtech
> > > 
> > > 
> > > ----- Original Message -----
> > > 
> > > > From: "Alexander Wels" <awels at redhat.com>
> > > > To: "Frantisek Kobzik" <fkobzik at redhat.com>
> > > > Cc: "Vojtech Szocs" <vszocs at redhat.com>
> > > > Sent: Monday, November 11, 2013 9:19:08 PM
> > > > Subject: Re: Weird behavior of multiple SetVmTicket query
> > > > 
> > > > Hi,
> > > > 
> > > > I did some debugging, and the problem is a race condition in the
> > > > MultipleActions class. The whole class seems to have a lot of
> > > > multi-threading
> > > > issues but if I modify the code to wait for the results of the actions
> > > > to
> > > > return before returning the return value, everything works fine.
> > > > 
> > > > I am attaching a patch that solves the issue at hand, but should not
> > > > be
> > > > considered a real patch. It is just to show the issue is in the
> > > > back-end
> > > > not
> > > > the front-end code. The front-end code is just exposing an issue in
> > > > the
> > > > back-
> > > > end
> > > > 
> > > > Alexander
> > > > 
> > > > On Monday, November 11, 2013 09:53:22 AM you wrote:
> > > > > Ok, thank you very much!
> > > > > 
> > > > > ----- Original Message -----
> > > > > From: "Alexander Wels" <awels at redhat.com>
> > > > > To: "Frantisek Kobzik" <fkobzik at redhat.com>
> > > > > Sent: Monday, November 11, 2013 2:15:43 PM
> > > > > Subject: Re: Weird behavior of multiple SetVmTicket query
> > > > > 
> > > > > Frantisek,
> > > > > 
> > > > > I had seen this before, let me test and fix it for you, it is very
> > > > > likely
> > > > > my
> > > > > patch broke that.
> > > > > 
> > > > > Alexander
> > > > > 
> > > > > On Monday, November 11, 2013 03:43:19 AM you wrote:
> > > > > > Hi Alex,
> > > > > > 
> > > > > > recently I noticed problems with invoking console for multiple VMs
> > > > > > (select
> > > > > > more VMs in webadmin and then hit the console btn). I was sure it
> > > > > > worked
> > > > > > in
> > > > > > past so i git-bisected the master branch and I discovered that
> > > > > > this
> > > > > > problem
> > > > > > is apparently caused by patch [1]. For single vm console
> > > > > > invocation
> > > > > > it
> > > > > > works fine, but for multiple VMs it doesn't.
> > > > > > 
> > > > > > I did some closer investigation with a debugger and it seems that
> > > > > > getSucceeded() on the return val of SetVmTicket command returns
> > > > > > false
> > > > > > in
> > > > > > case of multiple execution despite the fact SetVmTicketCommand
> > > > > > sets
> > > > > > this
> > > > > > value to true. I suppose there is some problem with propagation of
> > > > > > command
> > > > > > return value from BE to FE. The same goes for the encapsulated
> > > > > > returnValue
> > > > > > attribute (it contains value of the generated ticket). When
> > > > > > invoking
> > > > > > multiple consoles it is null (although it has been filled on
> > > > > > backend).
> > > > > > Weird.
> > > > > > 
> > > > > > Tomas told me you did some optimizations for multiple command
> > > > > > executions,
> > > > > > do you know it might cause it? Please let me know if you have any
> > > > > > idea...
> > > > > > 
> > > > > > Cheers,
> > > > > > F.
> > > > > > 
> > > > > > [1]: http://gerrit.ovirt.org/#/c/17356/
> > > 
> > > _______________________________________________
> > > Engine-devel mailing list
> > > Engine-devel at ovirt.org
> > > http://lists.ovirt.org/mailman/listinfo/engine-devel
> > 
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