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https://issues.apache.org/jira/browse/ZOOKEEPER-1177?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13103183#comment-13103183
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Vishal Kathuria commented on ZOOKEEPER-1177:
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Thanks for reviewing and your comments Mahadev. I was leaning towards 1 as well.
There is one issue I am worried about if we use czxid as the index into the
bitmap - it will keep growing over time as the recently created nodes will have
a the latest zxid as the value. The bitmap will become sparse and we will need
some sort of bitmap compression scheme.
The bitmap compression would increase the cost of bit lookup.
I had another thought about allocating an id for a znode without causing a
change in the storage format.
We could allocate the node id for a znode as we are deserializing the data tree
during restore at startup time. All these ids would be sequential so the max
value of the node id would be the same as the total number of nodes.
For the new node create after startup, we can keep allocating node ids using
the same counter.
The only other thing we will have to design for is reusing node ids as the
znodes are deleted and created - this should be easy by keeping a bitmap of
free node id values.
Thanks
> Enabling a large number of watches for a large number of clients
> ----------------------------------------------------------------
>
> Key: ZOOKEEPER-1177
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1177
> Project: ZooKeeper
> Issue Type: Improvement
> Components: server
> Affects Versions: 3.3.3
> Reporter: Vishal Kathuria
> Assignee: Vishal Kathuria
> Fix For: 3.5.0
>
>
> In my ZooKeeper, I see watch manager consuming several GB of memory and I dug
> a bit deeper.
> In the scenario I am testing, I have 10K clients connected to an observer.
> There are about 20K znodes in ZooKeeper, each is about 1K - so about 20M data
> in total.
> Each client fetches and puts watches on all the znodes. That is 200 million
> watches.
> It seems a single watch takes about 100 bytes. I am currently at 14528037
> watches and according to the yourkit profiler, WatchManager has 1.2 G
> already. This is not going to work as it might end up needing 20G of RAM just
> for the watches.
> So we need a more compact way of storing watches. Here are the possible
> solutions.
> 1. Use a bitmap instead of the current hashmap. In this approach, each znode
> would get a unique id when its gets created. For every session, we can keep
> track of a bitmap that indicates the set of znodes this session is watching.
> A bitmap, assuming a 100K znodes, would be 12K. For 10K sessions, we can keep
> track of watches using 120M instead of 20G.
> 2. This second idea is based on the observation that clients watch znodes in
> sets (for example all znodes under a folder). Multiple clients watch the same
> set and the total number of sets is a couple of orders of magnitude smaller
> than the total number of znodes. In my scenario, there are about 100 sets. So
> instead of keeping track of watches at the znode level, keep track of it at
> the set level. It may mean that get may also need to be implemented at the
> set level. With this, we can save the watches in 100M.
> Are there any other suggestions of solutions?
> Thanks
>
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