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From Gopal Vijayaraghavan <>
Subject Re: Hive +Tez+LLAP does not have obvious performance improvement than HIVE + Tez
Date Wed, 22 Nov 2017 20:03:11 GMT

> With these configurations,  the cpu utilization of llap is very low.

Low CPU usage has been observed with LLAP due to RPC starvation.

I'm going to assume that the build you're testing is a raw Hadoop 2.7.3 with no additional

Hadoop-RPC is single-threaded & has a single mutex lock in the 2.7.x branch, which is
fixed in 2.8.

Can you confirm if you have backported either

to your Hadoop implementation?

The secondary IO starvation comes from a series of HDFS performance problems which are easily
worked around. Here's the root-cause

The current workaround is to make sure that the HDFS & Hive user has a limits.d entry
to allow it to open a large number of sockets (which are fds).

This increases the FD limit for Hive & HDFS users (YARN also needs it, in case of Tez
due to shuffle being served out of the NodeManager).

After increasing the FDs, LLAP is fast enough to run through 128 socket openings within the
Linux TCP MSL (60 seconds)

The RHEL default for somaxconn  is 128, which causes 120s timeouts when HDFS silently loses
packets & forces the packet timeout to expire before retrying.

To know whether the problem has already happened, check the SNMP traps

# netstat -s | grep "overflow"

<n> times the listen queue of a socket overflowed

Or to know when the SYN flood issue has been worked around by the kernel with cookies.

# dmesg | grep cookies

After this, you get hit by the DNS starvation within LLAP where the DNS server traffic (port
53 UDP) gets lost (or the DNS server bans an IP due to massive number of packets).

This is a JDK internal detail, which ignores the DNS actual TTL values, which can be worked
around by running nscd or sssd on the host to cache dns lookups without generating UDP network
packets constantly.

If you need more detail on any of these, ask away. I've had to report and get backports for
several of these issues into HDP (mostly because perf issues are not generally community backports
& whatever has good workarounds remain off the priority lists).


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