KAFKA-6343 Documentation update in OS-level tuning section: add vm.ma… (#4358)

Reviewers: Manikumar Reddy O <manikumar.reddy@gmail.com>, Ismael Juma <ismael@juma.me.uk>, Jun Rao <junrao@gmail.com>

docs/ops.html

@@ -705,10 +705,11 @@
   <p>
   We have seen a few issues running on Windows and Windows is not currently a well supported platform though we would be happy to change that.
   <p>
-  It is unlikely to require much OS-level tuning, but there are two potentially important OS-level configurations:
+  It is unlikely to require much OS-level tuning, but there are three potentially important OS-level configurations:
   <ul>
-      <li>File descriptor limits: Kafka uses file descriptors for log segments and open connections.  If a broker hosts many partitions, consider that the broker needs at least (number_of_partitions)*(partition_size/segment_size) to track all log segments in addition to the number of connections the broker makes.  We recommend at least 100000 allowed file descriptors for the broker processes as a starting point.
+      <li>File descriptor limits: Kafka uses file descriptors for log segments and open connections.  If a broker hosts many partitions, consider that the broker needs at least (number_of_partitions)*(partition_size/segment_size) to track all log segments in addition to the number of connections the broker makes. We recommend at least 100000 allowed file descriptors for the broker processes as a starting point. Note: The mmap() function adds an extra reference to the file associated with the file descriptor fildes which is not removed by a subsequent close() on that file descriptor. This reference is removed when there are no more mappings to the file.
       <li>Max socket buffer size: can be increased to enable high-performance data transfer between data centers as <a href="http://www.psc.edu/index.php/networking/641-tcp-tune">described here</a>.
+      <li>Maximum number of memory map areas a process may have (aka vm.max_map_count). <a href="http://kernel.org/doc/Documentation/sysctl/vm.txt">See the Linux kernel documentation</a>. You should keep an eye at this OS-level property when considering the maximum number of partitions a broker may have. By default, on a number of Linux systems, the value of vm.max_map_count is somewhere around 65535. Each log segment, allocated per partition, requires a pair of index/timeindex files, and each of these files consumes 1 map area. In other words, each log segment uses 2 map areas. Thus, each partition requires minimum 2 map areas, as long as it hosts a single log segment. That is to say, creating 50000 partitions on a broker will result allocation of 100000 map areas and likely cause broker crash with OutOfMemoryError (Map failed) on a system with default vm.max_map_count. Keep in mind that the number of log segments per partition varies depending on the segment size, load intensity, retention policy and, generally, tends to be more than one.
   </ul>
   <p>