xx-use_method_fiddle.asciidoc

Hadoop Metrics

The USE Method applied to Hadoop

There’s an excellent methodology for performance analysis known as the "USE Method": For every resource, check Utilization, Saturation and Errors" ^[1]:

• utilization — How close to capacity is the resource? (ex: CPU usage, % disk used)

• saturation  — How often is work waiting? (ex: network drops or timeouts)

• errors  — Are there error events?

For example, USE metrics for a supermarket cashier would include:

• checkout utilization: flow of items across the belt; constantly stopping to look up the price of tomatoes or check coupons will harm utilization.

• checkout saturation: number of customers waiting in line

• checkout errors: calling for the manager

The cashier is an I/O resource; there are also capacity resources, such as available stock of turkeys:

• utilization: amount of remaining stock (zero remaining would be 100% utilization)

• saturation: in the US, you may need to sign up for turkeys near the Thanksgiving holiday.

• errors: spoiled or damaged product

As you can see, it’s possible to have high utilization and low saturation (steady stream of customers but no line, or no turkeys in stock but people happily buying ham), low utilization and high saturation (a cashier in training with a long line), or any other combination.

Why it’s interesting

It may not be obvious why the USE method is novel — "Hey, it’s good to record metrics" isn’t profound advice.

The USE method novel in its negative space — "Hey, it’s good enough to record this limited set of metrics" is quite surprising advice.

So if you prefer, here’s "USE method the extended remix": For each resource, check Utilization, Saturation and Errors. This is the necessary and sufficient foundation of a system performance analysis, and advanced effort is only justifiable once you have done so. There are further benefits:

• An elaborate solution space becomes a finite, parallelizable, delegatable list of activities.

• Blind spots become obvious. We once had a client issue where datanodes would go dark in difficult-to-reproduce circumstances. After much work, we found that persistent connections from Flume and chatter around numerous small files created by Hive were consuming all available datanode handler threads. We learned by pain what we could have learned by making a list — there was no visibility for the number of available handler threads.

• Saturation metrics are often non-obvious, and high saturation can have non-obvious consequences. (For example, the hard lower bound of TCP throughput)

• It’s known that Teddy Bears make superb level-1 support techs, because being forced to deliver a clear, uninhibited problem statement often suggests its solution. To some extent any framework this clear and simple will carry benefits, simply by forcing an organized problem description.

Look for the Bounding Resource

The USE metrics described below help you to identify the limiting resource of a job; to diagnose a faulty or misconfigured system; or to guide tuning and provisioning of the base system.

Improve / Understand Job Performance

Hadoop is designed to drive max utilization for its bounding resource by coordinating manageable saturation_ of the resources in front of it.

The "bounding resource" is the fundamental limit on performance — you can’t process a terabyte of data from disk faster than you can read a terabyte of data from disk. k

• disk read throughput

• disk write throughput

• job process CPU

• child process RAM, with efficient utilization of internal buffers

• If you don’t have the ability to specify hardware, you may need to accept "network read/write throughput" as a bounding resource during replication.

At each step of a job, what you’d like to see is very high utilization of exactly one bounding resource from that list, with reasonable headroom and managed saturation for everything else. What’s "reasonable"? As a rule of thumb, utilization above 70% in a non-bounding resource deserves a closer look.

Diagnose Flaws

Balanced Configuration/Provisioning of base system

Resource List

Please see the [glossary] for definitions of terms. I’ve borrowed many of the system-level metrics from Brendan Gregg’s Linux Checklist; visit there for a more-detailed list.

Table 1. USE Method Checklist

resource	type	metric	instrument
CPU-like concerns
CPU	utilization	system CPU	top/htop: CPU %, overall
	utilization	job process CPU	top/htop: CPU %, each child process
	saturation	max user processes	ulimit -u ("noproc" in /etc/security/limits.d/…​)
mapper slots	utilization	mapper slots used	jobtracker console; impacted by mapred.tasktracker.map.tasks.maximum
mapper slots	saturation	mapper tasks waiting	jobtracker console; impacted by scheduler and by speculative execution settings
	saturation	task startup overhead	???
	saturation	combiner activity	jobtracker console (TODO table cell name)
reducer slots	utilization	reducer slots used	jobtracker console; impacted by mapred.tasktracker.reduce.tasks.maximum
reducer slots	saturation	reducer tasks waiting	jobtracker console; impacted by scheduler and by speculative execution and slowstart settings
Memory concerns	_	__	_
memory capacity	utilization	total non-OS RAM	free, htop; you want the total excluding caches+buffers.
	utilization	child process RAM	free, htop: "RSS"; impacted by mapred.map.child.java.opts and mapred.reduce.child.java.opts
	utilization	JVM old-gen used	JMX
	utilization	JVM new-gen used	JMX
memory capacity	saturation	swap activity	vmstat 1 - look for "r" > CPU count.
	saturation	old-gen gc count	JMX, gc logs (must be specially enabled)
	saturation	old-gen gc pause time	JMX, gc logs (must be specially enabled)
	saturation	new-gen gc pause time	JMX, gc logs (must be specially enabled)
mapper sort buffer	utilization	record size limit	announced in job process logs; controlled indirectly by io.sort.record.percent, spill percent tunables
	utilization	record count limit	announced in job process logs; controlled indirectly by io.sort.record.percent, spill percent tunables
mapper sort buffer	saturation	spill count	spill counters (jobtracker console)
	saturation	sort streams	io sort factor tunable (io.sort.factor)
shuffle buffers	utilization	buffer size	child process logs
	utilization	buffer %used	child process logs
shuffle buffers	saturation	spill count	spill counters (jobtracker console)
	saturation	sort streams	merge parallel copies tunable mapred.reduce.parallel.copies (TODO: also io.sort.factor?)
OS caches/buffers	utilization	total c+b	free, htop
disk concerns	_	__	_
system disk I/O	utilization	req/s, read	iostat -xz 1 (system-wide); iotop (per process); /proc/{PID}/sched "se.statistics.iowait_sum"
	utilization	req/s, write	iostat -xz 1 (system-wide); iotop (per process); /proc/{PID}/sched "se.statistics.iowait_sum"
	utilization	MB/s, read	iostat -xz 1 (system-wide); iotop (per process); /proc/{PID}/sched "se.statistics.iowait_sum"
	utilization	MB/s, write	iostat -xz 1 (system-wide); iotop (per process); /proc/{PID}/sched "se.statistics.iowait_sum"
system disk I/O	saturation	queued requests	iostat -xnz 1; look for "avgqu-sz" > 1, or high "await".
system disk I/O	errors		/sys/devices/…/ioerr_cnt; smartctl, /var/log/messages
network concerns	_	__	_
network I/O	utilization		netstat; ip -s {link}; /proc/net/{dev} — RX/TX throughput as fraction of max bandwidth
network I/O	saturation		ifconfig ("overruns", "dropped"); netstat -s ("segments retransmited"); /proc/net/dev (RX/TX "drop")
network I/O	errors	interface-level	ifconfig ("errors", "dropped"); netstat -i ("RX-ERR"/"TX-ERR"); /proc/net/dev ("errs", "drop")
		request timeouts	daemon and child process logs
handler threads	utilization	nn handlers	(TODO: how to measure) vs dfs.namenode.handler.count
	utilization	jt handlers	(TODO: how to measure) vs
	utilization	dn handlers	(TODO: how to measure) vs dfs.datanode.handler.count
	utilization	dn xceivers	(TODO: how to measure) vs `dfs.datanode.max.xcievers
framework concerns	_	__	_
disk capacity	utilization	system disk used	df -bM
	utilization	HDFS directories	du -smc /path/to/mapred_scratch_dirs (for all directories in dfs.data.dir, dfs.name.dir, fs.checkpoint.dir)
	utilization	mapred scratch space	du -smc /path/to/mapred_scratch_dirs (TODO scratch dir tunable)
	utilization	total HDFS free	namenode console
	utilization	open file handles	ulimit -n ("nofile" in /etc/security/limits.d/…​)
job process	errors		stderr log
	errors		stdout log
	errors		counters
datanode	errors
namenode	errors
secondarynn	errors
tasktracker	errors
jobtracker	errors

Metrics in bold are critical resources — you would like to have exactly one of these at its full sustainable level

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1. developed by Brendan Gregg for system performance tuning, modified here for Hadoop