1HWLATDETECT(8) System Manager's Manual HWLATDETECT(8)
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6 hwlatdetect - program to control the ftrace kernel hardware latency
7 detection
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10 hwlatdetect [ --duration=<time> ] [--threshold=<usecs> ] [--win‐
11 dow=<time interval> ] [--width=<time interval> ]
12 [--hardlimit=<microsecond value> ] [--report=<path> ] [--debug ]
13 [--quiet ] [--watch]
14
17 hwlatdetect is a program that controls the ftrace kernel hardware
18 latency detector (hwlatdetector).hwlatdetector is used to detect large
19 system latencies induced by the behavior of certain underlying hardware
20 or firmware, independent of Linux itself. The code was developed orig‐
21 inally to detect SMIs (System Management Interrupts) on x86 systems,
22 however there is nothing x86 specific about it. It was originally
23 written for use by the "RT" patch set since the Real Time kernel is
24 highly latency sensitive.
25 SMIs are usually not serviced by the Linux kernel, which typically does
27 not even know that they are occurring. SMIs are instead are set up by
28 BIOS code and are serviced by BIOS code, usually for "critical" events
29 such as management of thermal sensors and fans. Sometimes though, SMIs
30 are used for other tasks and those tasks can spend an inordinate amount
31 of time in the handler (sometimes measured in milliseconds). Obviously
32 this is a problem if you are trying to keep event service latencies
33 down in the microsecond range.
34 The ftrace hardware latency detector works by hogging all of the cpus
36 for configurable amounts of time (by calling stop_machine()), polling
37 the CPU Time Stamp Counter for some period, then looking for gaps in
38 the TSC data. Any gap indicates a time when the polling was interrupted
39 and since the machine is stopped and interrupts turned off the only
40 thing that could do that would be an SMI.
41 The hwlatdetector script manages the mounting/unmounting of the debugfs
43 as well as interacting with the ftrace hwlatdetector If the debugfs is
44 already mounted then hwlatdetector will not unmount it after a run.
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48 --duration=<time>{s,m,d}
49 Run the detector logic in for the specified duration. The dura‐
50 tion is a base 10 integer number that defaults to a value in
51 seconds. An optional suffix may be specified to indicate min‐
52 utes, hours or days.
53 --threshold=<microsecond value>
56 Specify the TSC gap used to detect an SMI. Any gap value greater
57 than <threshold> is considered to be the result of an SMI occur‐
58 ring.
59 --hardlimit=<microsecond value>
62 The test is considered to fail if a value above the hardlimit
63 occurs. This affects the exit value of hwlatdetect
64 --window=<time value>{us,ms,s,m,d}
67 specify the size of the sample window. Converted to microseconds
68 when passed to the kernel.
69 --width=<time value>{us,ms,s,m,d}
72 The amount of time within the sample window where the detector
73 is actually sampling. Must be less than the --window value.
74 --report=FILENAME
76 Specify the output filename of the detector report. Default
77 behavior is to print to standard output
78 --debug
80 Turn on debug prints
81 --quiet
83 Turn off all information prints
84 --watch
86 print sample data to stdout as it arrives
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90 hwlatdetect was written by Clark Williams <williams@redhat.com>
91 hwlat_detector.ko was written by Jon Masters <jcm@redhat.com>
92 May 12, 2009 HWLATDETECT(8)