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From Jeremy Stribling <st...@nicira.com>
Subject Re: adding a separate thread to detect network timeouts faster
Date Tue, 10 Sep 2013 21:05:23 GMT
Good suggestion, thanks.  At the very least, I think what we have in 
mind would be off by default, so users could only turn it on if they 
know they have relatively few clients and slow disks.  An adaptive 
scheme would be even better, obviously.

On 09/10/2013 02:04 PM, Ted Dunning wrote:
> Perhaps you should be suggesting a design that is adaptive rather than 
> configured and guarantees low overhead at the cost of notification 
> time in extreme scenarios.
> For instance, the server can send no more than 1000 (or whatever 
> number) HB's per second and never more than one per second to any 
> client.  This caps the cost nicely.
> On Tue, Sep 10, 2013 at 1:59 PM, Ted Dunning <ted.dunning@gmail.com 
> <mailto:ted.dunning@gmail.com>> wrote:
>     Since you are talking about client connection failure detection,
>     no, I don't think that there is a major barrier other than
>     actually implementing a reliable check.
>     Keep in mind the cost.  There are ZK installs with 100,000
>     clients.  If these are heartbeating every 2 seconds, you have
>     50,000 packets per second hitting the quorum or 10,000 per server
>     if all connections are well balanced.
>     If you only have 10 clients, the network burden is nominal.
>     On Tue, Sep 10, 2013 at 1:34 PM, Jeremy Stribling
>     <strib@nicira.com <mailto:strib@nicira.com>> wrote:
>         I mostly agree, but let's assume that a ~5x speedup in
>         detecting those types of failures is considered significant
>         for some people. Are there technical reasons that would
>         prevent this idea from working?
>         On 09/10/2013 01:31 PM, Ted Dunning wrote:
>             I don't see the strong value here.  A few failures would
>             be detected more
>             quickly, but I am not convinced that this would actually
>             improve
>             functionality significantly.
>             On Tue, Sep 10, 2013 at 1:01 PM, Jeremy Stribling
>             <strib@nicira.com <mailto:strib@nicira.com>> wrote:
>                 Hi all,
>                 Let's assume that you wanted to deploy ZK in a
>                 virtualized environment,
>                 despite all of the known drawbacks.  Assume we could
>                 deploy it such that
>                 the ZK servers were all using independent CPUs and
>                 storage (though not
>                 dedicated disks).  Obviously, the shared disks (shared
>                 with other, non-ZK
>                 VMs on the same hypervisor) will cause ZK to hit the
>                 default session
>                 timeout occasionally, so you would need to raise the
>                 existing session
>                 timeout to something like 30 seconds.
>                 I'm curious if there would be any technical drawbacks
>                 to adding an
>                 additional heartbeat mechanism between the clients and
>                 the servers, which
>                 would have the goal of detecting network-only failures
>                 faster than the
>                 existing heartbeat mechanism.  The idea is that there
>                 would be a new thread
>                 dedicated to processing these heartbeats, which would
>                 not get blocked on
>                 I/O.  Then the clients could configure a second,
>                 smaller timeout value, and
>                 it would be assumed that any such timeout indicated a
>                 real problem.  The
>                 existing mechanism would still be in place to catch
>                 I/O-related errors.
>                 I understand the philosophy that there should be some
>                 heartbeat mechanism
>                 that takes the disk into account, but I'm having
>                 trouble coming up with
>                 technical reasons not to add a second mechanism.
>                 Obviously, the advantage
>                 would be that the clients could detect network
>                 failures and system crashes
>                 more quickly in an environment with slow disks, and
>                 fail over to other
>                 servers more quickly.  The only disadvantages I can
>                 come up with are:
>                 1) More code complexity, and slightly more heartbeat
>                 traffic on the wire
>                 2) I think the servers have to log session expirations
>                 to disk, so if the
>                 sessions expire at a faster rate than the disk can
>                 handle, it might lead to
>                 a large backlog.
>                 Are there other drawbacks I am missing?  Would a patch
>                 that added
>                 something like this be considered, or is it dead from
>                 the start? Thanks,
>                 Jeremy

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