The element in InvertedIndexSearcherCache is inverted index searcher, which is a file descriptor of inverted index file, so InvertedIndexSearcherCache is actually cache file descriptor of inverted index file.
If open file descriptor limit of the Linux system is set too small and config inverted_index_searcher_cache_limit is too big, during high pressure load maybe cause "Too many open files".
So, when insert inverted index searcher into InvertedIndexSearcherCache, need also check whether reach file_descriptor_number limit for inverted index file.
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