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https://issues.apache.org/jira/browse/ZOOKEEPER-1177?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13177492#comment-13177492
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Patrick Hunt commented on ZOOKEEPER-1177:
-----------------------------------------

In reviewing this code I noticed a couple concerns:

1) it's unfortunate there is no SparseBitSet, in the worst case scenario we 
might end up wasting memory due to the id assignment. Say you had 1k sessions 
and 1m znodes. If the first session set watches on all 1m znodes and the other 
999 sessions set a watch on znode 1m (the one with the highest id), the 999 
sessions would have empty bitsets, save for the very last bit (#1m). Not sure 
if this is a real concern in practice though.

2) more concerning is removeWatcher - it removes the bitset, but not the 
associated entries in path2Id/id2Path. Granted cleanup is an expensive 
operation in this case, however without it we'll end up "leaking" 
path2id/id2path mappings in some cases.

1 doesn't seem like a blocker given the benefits, perhaps 2 is not a concern 
given in most cases the set of paths is bounded and if the znode is deleted the 
watch manager will cleanup at that time.

Thoughts?
                
> 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
>
>         Attachments: ZooKeeper-with-fix-for-findbugs-warning.patch, 
> ZooKeeper.patch, Zookeeper-after-resolving-merge-conflicts.patch
>
>
> 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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