Commits (with openSearcher = true) are too slow in solr 8

[raj.yadav]

Hi everyone,
We have two parallel system one is  solr 8.5.2 and other one is solr 5.4
In solr_5.4 commit time with opensearcher true is 10 to 12 minutes while in
solr_8 it's around 25 minutes.

This is our current caching policy of solr_8

<filterCache class="solr.CaffeineCache"
                 size="32768"
                 initialSize="6000"
                 autowarmCount="6000"/>

    <queryResultCache class="solr.CaffeineCache"
                      size="25600"
                      initialSize="6000"
                      autowarmCount="0"/>

    <documentCache class="solr.CaffeineCache"
                   size="32768"
                   initialSize="6144"
                   autowarmCount="0"/>

In solr 5, we are using FastLRUCache (instead of CaffeineCache) and other
parameters are same.

While debugging this we came across this page.
https://cwiki.apache.org/confluence/display/SOLR/SolrPerformanceProblems#SolrPerformanceProblems-Slowcommits

Here one of the reasons for slow commit is mentioned as:
*/`Heap size issues. Problems from the heap being too big will tend to be
infrequent, while problems from the heap being too small will tend to happen
consistently.`/*

Can anyone please help me understand the above point?

System config:
disk size: 250 GB
cpu: (8 vcpus, 64 GiB memory)
Index size: 11 GB
JVM heap size: 30 GB



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