Large ring_creation_size

Jon Meredith jmeredith at basho.com
Thu Apr 14 14:00:58 EDT 2011


Hi Greg,

I played with this a little last night and this morning and I can reproduce
the behavior you are seeing - my two nodes ate more than a combined 15gig of
memory with 16384 partitions and were promptly killed by the O/S.

I haven't had a chance to analyze yet, so this is pure speculation. I
suspect that implementations of functions for calculating partition owners,
preference lists and which partitions to take when nodes join perform
acceptably for the <= 1024 partition case but blow up in some way beyond
that. I was hoping that 4096 would work for you, but it sounds like that has
problems too.

I'm finishing up some high priority work at the moment but will investigate
as time permits.

Jon

On Thu, Apr 14, 2011 at 11:48 AM, Greg Nelson <grourk at dropcam.com> wrote:

>  We have a exact idea of the amount of data we'll be storing, and the
> kinds of machines we'll be storing them on.  The simple math of (total data
> we'll be storing 6 months from now) / (total capacity of a single node) *
> (number of duplicates of each datum we'd like to store for redundancy) gives
> us a number a concrete number of machines we'll need, whether we're using
> Riak or something else...
>
> However, that's a little off track from the question I'm trying to answer.
>  Which is:
>
> Why is it when I start two nodes with a large-ish ring_creation_size -- as
> soon as I join the second node to the first, CPU and memory usage of both
> nodes goes through the roof?  No data stored yet.  This even happens with a
> ring_creation_size I wouldn't consider huge, like 4096.
>
> So that is the small configuration I'm starting with and that is the limit
> I'm hitting.
>
> I realize that the feasibility of building out a single 1000 node cluster
> is a larger question.  But I can tell you we'll get there; having to shard
> across 10 100-node clusters is an option.  Regardless, I'd like to have an
> understanding of what the resource overhead of each vnode is...
>
> On Thursday, April 14, 2011 at 6:38 AM, Sean Cribbs wrote:
>
> Good points, Dave.
>
> Also, it's worth mentioning that we've seen that many customers and
> open-source users think they will need many more nodes than they actually
> do. Many are able to start with 5 nodes and are happy for quite a while.
>  The only way to tell what you actually need is to start with a baseline
> configuration and simulate some percentage above your current load.  Once
> you've figured out what size that initial cluster is, start with (number of
> nodes) * 50 as the ring_creation_size (rounded to the nearest power of 2 of
> course).  This gives you a growth factor of about 5 before you need to
> consider changing.
>
> As well, there's some ops "common sense" that says the lifetime of any
> single architecture is 18 months or less.  That doesn't necessarily mean
> that you'll be building a new cluster with a larger ring size in 18 months,
> but just that your needs will be different at that time and are hard to
> predict.  Plan for now, worry about the 1000 node cluster when you actually
> need it.
>
> Sean Cribbs <sean at basho.com>
> Developer Advocate
> Basho Technologies, Inc.
> http://basho.com/
>
> On Apr 14, 2011, at 9:09 AM, Dave Barnes wrote:
>
> Sorry I feel compelled to chime in.
>
> Maybe you could assess your physical node limits and start with a small
> configuration, then increase  it and increase it until you hit a limit.
>
> Work small to large.
>
> Once you find the pain point, lets us know what resource ran out.
>
> You will learn a lot along the way on how your servers behave and we'll
> discover a lot when you share the results.
>
> Thanks for digging in,
>
> Dave
>
> On Wed, Apr 13, 2011 at 5:11 PM, Greg Nelson <grourk at dropcam.com> wrote:
>
>  Ok, how about in this case I described?  It runs out of memory with a
> single pair of nodes...
>
> (Or did you mean there's a connection between each pair of vnodes?)
>
> On Wednesday, April 13, 2011 at 1:56 PM, Jon Meredith wrote:
>
> Hi Greg et al,
>
> As you say largest known is not largest possible.  Internally within Basho,
> the largest cluster we've experimented with so far had 50 nodes.
>
> Going beyond that it's speculation from me about pain points.
>
> 1) It is true that you need enough file descriptors to start up all
> partitions when a node restarts - Riak checks if there is any handoff data
> pending for each partition.  We have work scheduled to address that in the
> medium term. The plan is to only spin up partitions the node owns and any
> that have been started as fallbacks that handoff has not completed for.
> Until that work is done you will need a high ulimit with large ring sizes.
>
> 2) It is also true that Erlang runs a fully connected network, so there
> will be connections between each node pair in the cluster.  We haven't
> determined the point at which it becomes a problem.
>
> So it looks like you'll be pushing the known limits.  Basho will do our
> very best to help overcome any obstacles as you encounter them.
>
> Jon Meredith
> Basho Technologies.
>
> On Wed, Apr 13, 2011 at 1:41 PM, Greg Nelson <grourk at dropcam.com> wrote:
>
>  The largest known riak cluster != the largest possible riak cluster.  ;-)
>
> The inter node communication of the cluster depends on the data set and
> usage pattern, doesn't it?  Or is there some constant overhead that tops out
> at a few hundred nodes?  I should point out that we'll have big data, but
> not a huge number of keys.
>
> The number of vnodes in the cluster should be equal to the
> ring_creation_size under normal circumstances, shouldn't it?  So when I have
> a one node cluster, that node is running ring_creation_size vnodes...  File
> descriptors probably isn't a problem -- these machines won't be doing
> anything else, and the limits are set to 65536.
>
> Thinking about the internode communication you mentioned, that's probably
> where the resource hog is..  socket buffers, etc.
>
> Anyway, I'd also love to hear more from basho.  :)
>
> On Wednesday, April 13, 2011 at 12:33 PM, siculars at gmail.com wrote:
>
> Ill just chime in and say that this is not practical for a few reasons. The
> largest known riak cluster has like 50 or 60 nodes. Afaik, inter node
> communication of erlang clusters top out at a few hundred nodes. I'm also
> under the impression that each physical node has to have enough file
> descriptors to accommodate every virtual node in the cluster.
>
> I'd love to hear more from basho.
>
> -alexander
>
>
> Sent from my Verizon Wireless BlackBerry
>
> -----Original Message-----
> From: Greg Nelson <grourk at dropcam.com>
> Sender: riak-users-bounces at lists.basho.com
> Date: Wed, 13 Apr 2011 12:13:34
> To: <riak-users at lists.basho.com>
> Subject: Large ring_creation_size
>
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