biolatency

README.md

@@ -64,6 +64,7 @@ Examples:
 
 Tools:
 
+- tools/[biolatency](tools/biolatency): Summarize block device I/O latency as a histogram. [Examples](tools/biolatency_example.txt).
 - tools/[biosnoop](tools/biosnoop): Trace block device I/O with PID and latency. [Examples](tools/biosnoop_example.txt).
 - tools/[funccount](tools/funccount): Count kernel function calls. [Examples](tools/funccount_example.txt).
 - tools/[funclatency](tools/funclatency): Time kernel functions and show their latency distribution. [Examples](tools/funclatency_example.txt).

man/man8/biolatency.8

+.TH biolatency 8  "2015-08-20" "USER COMMANDS"
+.SH NAME
+biolatency \- Summarize block device I/O latency as a histogram.
+.SH SYNOPSIS
+.B biolatency [\-h] [\-T] [\-Q] [\-m] [interval [count]]
+.SH DESCRIPTION
+biolatency traces block device I/O (disk I/O), and records the distribution
+of I/O latency (time). This is printed as a histogram either on Ctrl-C, or
+after a given interval in seconds.
+
+The latency of the disk I/O is measured from the issue to the device to its
+completion. A \-Q option can be used to include time queued in the kernel.
+
+This tool uses in-kernel eBPF maps for storing timestamps and the histogram,
+for efficiency.
+
+This works by tracing various kernel blk_*() functions using dynamic tracing,
+and will need updating to match any changes to these functions.
+
+Since this uses BPF, only the root user can use this tool.
+.SH REQUIREMENTS
+CONFIG_BPF and bcc.
+.SH OPTIONS
+\-h
+Print usage message.
+.TP
+\-T
+Include timestamps on output.
+.TP
+\-m
+Output histogram in milliseconds.
+.TP
+interval
+Output interval, in seconds.
+.TP
+count
+Number of outputs.
+.SH EXAMPLES
+.TP
+Summarize block device I/O latency as a histogram:
+#
+.B biolatency
+.TP
+Print 1 second summaries, 10 times:
+#
+.B biolatency 1 10
+.TP
+Print 1 second summaries, using milliseconds as units for the histogram, and
+include timestamps on output:
+#
+.B biolatency \-mT 1
+.TP
+Include OS queued time in I/O time:
+#
+.B biolatency \-Q
+.SH FIELDS
+.TP
+usecs
+Microsecond range
+.TP
+mecs
+Millisecond range
+.TP
+count
+How many I/O fell into this range
+.TP
+distribution
+An ASCII bar chart to visualize the distribution (count column)
+.SH OVERHEAD
+This traces kernel functions and maintains in-kernel timestamps and a histgroam,
+which are asynchronously copied to user-space. This method is very efficient,
+and the overhead for most storage I/O rates (< 10k IOPS) should be negligible.
+If you have a higher IOPS storage environment, test and quantify the overhead
+before use.
+.SH SOURCE
+This is from bcc.
+.IP
+https://github.com/iovisor/bcc
+.PP
+Also look in the bcc distribution for a companion _examples.txt file containing
+example usage, output, and commentary for this tool.
+.SH OS
+Linux
+.SH STABILITY
+Unstable - in development.
+.SH AUTHOR
+Brendan Gregg
+.SH SEE ALSO
+biosnoop(8)

tools/biolatency

+#!/usr/bin/python
+#
+# biolatency	Summarize block device I/O latency as a histogram.
+#		For Linux, uses BCC, eBPF.
+#
+# USAGE: biolatency [-h] [-T] [-Q] [-m] [interval] [count]
+#
+# Copyright (c) 2015 Brendan Gregg.
+# Licensed under the Apache License, Version 2.0 (the "License")
+#
+# 20-Sep-2015	Brendan Gregg	Created this.
+
+from __future__ import print_function
+from bcc import BPF
+from time import sleep, strftime
+import argparse
+
+# arguments
+examples = """examples:
+    ./biolatency            # summarize block I/O latency as a histogram
+    ./biolatency 1 10       # print 1 second summaries, 10 times
+    ./biolatency -mT 1      # 1s summaries, milliseconds, and timestamps
+    ./biolatency -Q         # include OS queued time in I/O time
+"""
+parser = argparse.ArgumentParser(
+	description="Summarize block device I/O latency as a histogram",
+	formatter_class=argparse.RawDescriptionHelpFormatter,
+	epilog=examples)
+parser.add_argument("-T", "--timestamp", action="store_true",
+	help="include timestamp on output")
+parser.add_argument("-Q", "--queued", action="store_true",
+	help="include OS queued time in I/O time")
+parser.add_argument("-m", "--milliseconds", action="store_true",
+	help="millisecond histogram")
+parser.add_argument("interval", nargs="?", default=99999999,
+	help="output interval, in seconds")
+parser.add_argument("count", nargs="?", default=99999999,
+	help="number of outputs")
+args = parser.parse_args()
+countdown = int(args.count)
+debug = 0
+
+# load BPF program
+bpf_text = """
+#include <uapi/linux/ptrace.h>
+#include <linux/blkdev.h>
+
+BPF_TABLE(\"array\", int, u64, dist, 64);
+BPF_HASH(start, struct request *);
+
+// time block I/O
+int trace_req_start(struct pt_regs *ctx, struct request *req)
+{
+	u64 ts = bpf_ktime_get_ns();
+	start.update(&req, &ts);
+	return 0;
+}
+
+// output
+int trace_req_completion(struct pt_regs *ctx, struct request *req)
+{
+	u64 *tsp, delta;
+
+	// fetch timestamp and calculate delta
+	tsp = start.lookup(&req);
+	if (tsp == 0) {
+		return 0;	// missed issue
+	}
+	delta = bpf_ktime_get_ns() - *tsp;
+	FACTOR
+
+	// store as histogram
+	int index = bpf_log2l(delta);
+	u64 *leaf = dist.lookup(&index);
+	if (leaf) (*leaf)++;
+
+	start.delete(&req);
+	return 0;
+}
+"""
+if args.milliseconds:
+	bpf_text = bpf_text.replace('FACTOR', 'delta /= 1000000;')
+	label = "msecs"
+else:
+	bpf_text = bpf_text.replace('FACTOR', 'delta /= 1000;')
+	label = "usecs"
+if debug:
+	print(bpf_text)
+
+# load BPF program
+b = BPF(text=bpf_text)
+if args.queued:
+	b.attach_kprobe(event="blk_account_io_start", fn_name="trace_req_start")
+else:
+	b.attach_kprobe(event="blk_start_request", fn_name="trace_req_start")
+	b.attach_kprobe(event="blk_mq_start_request", fn_name="trace_req_start")
+b.attach_kprobe(event="blk_account_io_completion",
+    fn_name="trace_req_completion")
+
+print("Tracing block device I/O... Hit Ctrl-C to end.")
+
+# output
+exiting = 0 if args.interval else 1
+dist = b.get_table("dist")
+while (1):
+	try:
+		sleep(int(args.interval))
+	except KeyboardInterrupt:
+		exiting=1
+
+	print()
+	if args.timestamp:
+		print("%-8s\n" % strftime("%H:%M:%S"), end="")
+
+	dist.print_log2_hist(label)
+	dist.clear()
+
+	countdown -= 1
+	if exiting or countdown == 0:
+		exit()

tools/biolatency_example.txt

+Demonstrations of biolatency, the Linux eBPF/bcc version.
+
+
+biolatency traces block device I/O (disk I/O), and records the distribution
+of I/O latency (time), printing this as a histogram when Ctrl-C is hit.
+For example:
+
+# ./biolatency 
+Tracing block device I/O... Hit Ctrl-C to end.
+^C
+     usecs           : count     distribution
+       0 -> 1        : 0        |                                      |
+       2 -> 3        : 0        |                                      |
+       4 -> 7        : 0        |                                      |
+       8 -> 15       : 0        |                                      |
+      16 -> 31       : 0        |                                      |
+      32 -> 63       : 0        |                                      |
+      64 -> 127      : 1        |                                      |
+     128 -> 255      : 12       |********                              |
+     256 -> 511      : 15       |**********                            |
+     512 -> 1023     : 43       |*******************************       |
+    1024 -> 2047     : 52       |**************************************|
+    2048 -> 4095     : 47       |**********************************    |
+    4096 -> 8191     : 52       |**************************************|
+    8192 -> 16383    : 36       |**************************            |
+   16384 -> 32767    : 15       |**********                            |
+   32768 -> 65535    : 2        |*                                     |
+   65536 -> 131071   : 2        |*                                     |
+
+The latency of the disk I/O is measured from the issue to the device to its
+completion. A -Q option can be used to include time queued in the kernel.
+
+This example output shows a large mode of latency from about 128 microseconds
+to about 32767 microseconds (33 milliseconds). The bulk of the I/O was
+between 1 and 8 ms, which is the expected block device latency for
+rotational storage devices.
+
+The highest latency seen while tracing was between 65 and 131 milliseconds:
+the last row printed, for which there were 2 I/O.
+
+For efficiency, biolatency uses an in-kernel eBPF map to store timestamps
+with requests, and another in-kernel map to store the histogram (the "count")
+column, which is copied to user-space only when output is printed. These
+methods lower the perormance overhead when tracing is performed.
+
+
+In the following example, the -m option is used to print a histogram using
+milliseconds as the units (which eliminates the first several rows), -T to
+print timestamps with the output, and to print 1 second summaries 5 times:
+
+# ./biolatency -mT 1 5
+Tracing block device I/O... Hit Ctrl-C to end.
+
+06:20:16
+     msecs           : count     distribution
+       0 -> 1        : 36       |**************************************|
+       2 -> 3        : 1        |*                                     |
+       4 -> 7        : 3        |***                                   |
+       8 -> 15       : 17       |*****************                     |
+      16 -> 31       : 33       |**********************************    |
+      32 -> 63       : 7        |*******                               |
+      64 -> 127      : 6        |******                                |
+
+06:20:17
+     msecs           : count     distribution
+       0 -> 1        : 96       |************************************  |
+       2 -> 3        : 25       |*********                             |
+       4 -> 7        : 29       |***********                           |
+       8 -> 15       : 62       |***********************               |
+      16 -> 31       : 100      |**************************************|
+      32 -> 63       : 62       |***********************               |
+      64 -> 127      : 18       |******                                |
+
+06:20:18
+     msecs           : count     distribution
+       0 -> 1        : 68       |*************************             |
+       2 -> 3        : 76       |****************************          |
+       4 -> 7        : 20       |*******                               |
+       8 -> 15       : 48       |*****************                     |
+      16 -> 31       : 103      |**************************************|
+      32 -> 63       : 49       |******************                    |
+      64 -> 127      : 17       |******                                |
+
+06:20:19
+     msecs           : count     distribution
+       0 -> 1        : 522      |*************************************+|
+       2 -> 3        : 225      |****************                      |
+       4 -> 7        : 38       |**                                    |
+       8 -> 15       : 8        |                                      |
+      16 -> 31       : 1        |                                      |
+
+06:20:20
+     msecs           : count     distribution
+       0 -> 1        : 436      |**************************************|
+       2 -> 3        : 106      |*********                             |
+       4 -> 7        : 34       |**                                    |
+       8 -> 15       : 19       |*                                     |
+      16 -> 31       : 1        |                                      |
+
+How the I/O latency distribution changes over time can be seen.
+
+
+
+The -Q option begins measuring I/O latency from when the request was first
+queued in the kernel, and includes queuing latency:
+
+# ./biolatency -Q
+Tracing block device I/O... Hit Ctrl-C to end.
+^C
+     usecs           : count     distribution
+       0 -> 1        : 0        |                                      |
+       2 -> 3        : 0        |                                      |
+       4 -> 7        : 0        |                                      |
+       8 -> 15       : 0        |                                      |
+      16 -> 31       : 0        |                                      |
+      32 -> 63       : 0        |                                      |
+      64 -> 127      : 0        |                                      |
+     128 -> 255      : 3        |*                                     |
+     256 -> 511      : 37       |**************                        |
+     512 -> 1023     : 30       |***********                           |
+    1024 -> 2047     : 18       |*******                               |
+    2048 -> 4095     : 22       |********                              |
+    4096 -> 8191     : 14       |*****                                 |
+    8192 -> 16383    : 48       |*******************                   |
+   16384 -> 32767    : 96       |**************************************|
+   32768 -> 65535    : 31       |************                          |
+   65536 -> 131071   : 26       |**********                            |
+  131072 -> 262143   : 12       |****                                  |
+
+This better reflects the latency suffered by the application (if it is
+synchronous I/O), whereas the default mode without kernel queueing better
+reflects the performance of the device.
+
+Note that the storage device (and storage device controller) usually have
+queues of their own, which are always included in the latency, with or
+without -Q.
+
+
+USAGE message:
+
+# ./biolatency -h
+usage: biolatency [-h] [-T] [-Q] [-m] [interval] [count]
+
+Summarize block device I/O latency as a histogram
+
+positional arguments:
+  interval            output interval, in seconds
+  count               number of outputs
+
+optional arguments:
+  -h, --help          show this help message and exit
+  -T, --timestamp     include timestamp on output
+  -Q, --queued        include OS queued time in I/O time
+  -m, --milliseconds  millisecond histogram
+
+examples:
+    ./biolatency            # summarize block I/O latency as a histogram
+    ./biolatency 1 10       # print 1 second summaries, 10 times
+    ./biolatency -mT 1      # 1s summaries, milliseconds, and timestamps
+    ./biolatency -Q         # include OS queued time in I/O time