Recently i upgraded to MariaDB 10.3.10 using directadmin custombuild function. And i noticed the mysql performance not very good. and while checking mysqltuner.pl suggestion, i found the number open file limit used extremely high.

-------- Storage Engine Statistics -----------------------------------------------------------------
[--] Status: +Aria +CSV +InnoDB +MEMORY +MRG_MyISAM +MyISAM +PERFORMANCE_SCHEMA +SEQUENCE
[--] Data in MyISAM tables: 78.0M (Tables: 5)
[--] Data in InnoDB tables: 21.1G (Tables: 236)
[--] Data in MEMORY tables: 10.8M (Tables: 3)
[!!] Total fragmented tables: 1

-------- Analysis Performance Metrics --------------------------------------------------------------
[--] innodb_stats_on_metadata: OFF
[OK] No stat updates during querying INFORMATION_SCHEMA.

-------- Security Recommendations ------------------------------------------------------------------
[OK] There are no anonymous accounts for any database users
[OK] All database users have passwords assigned
[!!] There is no basic password file list!

-------- CVE Security Recommendations --------------------------------------------------------------
[--] Skipped due to --cvefile option undefined

-------- Performance Metrics -----------------------------------------------------------------------
[--] Up for: 14h 11m 59s (52M q [1K qps], 3M conn, TX: 1503G, RX: 15G)
[--] Reads / Writes: 75% / 25%
[--] Binary logging is disabled
[--] Physical Memory : 94.2G
[--] Max MySQL memory : 37.4G
[--] Other process memory: 611.8M
[--] Total buffers: 35.1G global + 2.9M per thread (620 max threads)
[--] P_S Max memory usage: 531M
[--] Galera GCache Max memory usage: 0B
[OK] Maximum reached memory usage: 35.7G (37.94% of installed RAM)
[OK] Maximum possible memory usage: 37.4G (39.68% of installed RAM)
[OK] Overall possible memory usage with other process is compatible with memory available
[OK] Slow queries: 0% (0/52M)
[OK] Highest usage of available connections: 7% (45/620)
[OK] Aborted connections: 0.00% (0/3451781)
[--] Skipped name resolution test due to skip_networking=ON in system variables.
[OK] Query cache is disabled by default due to mutex contention on multiprocessor machines.
[OK] Sorts requiring temporary tables: 0% (78 temp sorts / 3M sorts)
[!!] Joins performed without indexes: 837224
[!!] Temporary tables created on disk: 50% (1M on disk / 3M total)
[OK] Thread cache hit rate: 99% (45 created / 3M connections)
[OK] Table cache hit rate: 90% (667 open / 735 opened)
*[!!] Open file limit used: 28147927174348900% (18446744073B/65K)
*[!!] Table locks acquired immediately: 86%

-------- Performance schema ------------------------------------------------------------------------
[--] Memory used by P_S: 531.1M
[--] Sys schema isn't installed.

-------- ThreadPool Metrics ------------------------------------------------------------------------
[--] ThreadPool stat is enabled.
[--] Thread Pool Size: 36 thread(s).
[--] Using default value is good enough for your version (10.3.10-MariaDB)

-------- MyISAM Metrics ----------------------------------------------------------------------------
[!!] Key buffer used: 19.5% (516M used / 2B cache)
[OK] Key buffer size / total MyISAM indexes: 2.5G/16.8M
[OK] Read Key buffer hit rate: 100.0% (98M cached / 10K reads)
[!!] Write Key buffer hit rate: 88.3% (7M cached / 6M writes)

-------- InnoDB Metrics ----------------------------------------------------------------------------
[--] InnoDB is enabled.
[--] InnoDB Thread Concurrency: 0
[OK] InnoDB File per table is activated
[OK] InnoDB buffer pool / data size: 32.0G/21.1G
[OK] Ratio InnoDB log file size / InnoDB Buffer pool size: 4.0G * 2/32.0G should be equal 25%
[OK] InnoDB buffer pool instances: 32
[--] Number of InnoDB Buffer Pool Chunk : 256 for 32 Buffer Pool Instance(s)
[OK] Innodb_buffer_pool_size aligned with Innodb_buffer_pool_chunk_size & Innodb_buffer_pool_instances
[OK] InnoDB Read buffer efficiency: 100.00% (36999275046 hits/ 37000117624 total)
[!!] InnoDB Write Log efficiency: 86.04% (3658360 hits/ 4251694 total)
[OK] InnoDB log waits: 0.00% (0 waits / 593334 writes)

-------- AriaDB Metrics ----------------------------------------------------------------------------
[--] AriaDB is enabled.
[OK] Aria pagecache size / total Aria indexes: 128.0M/1B
[OK] Aria pagecache hit rate: 98.5% (106M cached / 1M reads)

-------- TokuDB Metrics ----------------------------------------------------------------------------
[--] TokuDB is disabled.

-------- XtraDB Metrics ----------------------------------------------------------------------------
[--] XtraDB is disabled.

-------- Galera Metrics ----------------------------------------------------------------------------
[--] Galera is disabled.

-------- Replication Metrics -----------------------------------------------------------------------
[--] Galera Synchronous replication: NO
[--] No replication slave(s) for this server.
[--] Binlog format: MIXED
[--] XA support enabled: ON
[--] Semi synchronous replication Master: OFF
[--] Semi synchronous replication Slave: OFF
[--] This is a standalone server

-------- Recommendations ---------------------------------------------------------------------------
General recommendations:
Run OPTIMIZE TABLE to defragment tables for better performance
OPTIMIZE TABLE admin_xxxxx.`xf_search`; – can free 37 MB
Total freed space after theses OPTIMIZE TABLE : 37 Mb
MySQL was started within the last 24 hours - recommendations may be inaccurate
Adjust your join queries to always utilize indexes
Temporary table size is already large - reduce result set size
Reduce your SELECT DISTINCT queries without LIMIT clauses
Optimize queries and/or use InnoDB to reduce lock wait
Consider installing Sys schema from https://github.com/mysql/mysql-sys
Variables to adjust:
join_buffer_size (> 256.0K, or always use indexes with JOINs)
open_files_limit (> 65535)