How to fix exceeded limit of maxWarmingSearchers?

As it's well explained here you should reduce the number of commits you make, or change the value of maxWarmingSearchers in solrconfig.xml (which is not a good practice)

As per the Solr FAQ: What does "exceeded limit of maxWarmingSearchers=X" mean?

If you encounter this error a lot, you can (in theory) increase the number in your maxWarmingSearchers, but that is risky to do unless you are confident you have the system resources (RAM, CPU, etc...) to do it safely. A more correct way to deal with the situation is to reduce how frequently you send commits.

What this error means is that you are basically making commits too often and the internal cache can't keep up with the frequency you are saying "clear the cache and let me search with the new data". You need to decrease the frequency you are making commits. You can find more information about this problem here in Near Realtime Search Tuning but the basic idea is the more facets you use the greater the interval you will need between commits.

One way I got around this was I stopped making manual commits (i.e. having my application submit data to solr and then executing a commit request) and turning on solr autocommit.

Here's an example:

<!-- solrconfig.xml -->
<autoCommit>
  <maxDocs>10000</maxDocs> <!-- maximum uncommited docs before autocommit triggered -->
  <maxTime>15000</maxTime> <!-- maximum time (in MS) after adding a doc before an autocommit is triggered -->
  <openSearcher>false</openSearcher> <!-- SOLR 4.0.  Optionally don't open a searcher on hard commit.  This is useful to minimize the size of transaction logs that keep track of uncommitted updates. -->
</autoCommit>

You will have to figure out how large of interval you will need (i.e. maxTime) but in practice every time I add more faceted search to my application (or more indexes or what have you) I have to increase the interval.

If you need more real time search than the frequency of these commits will allow you can look into Solr soft commits.