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authorArtem Bityutskiy <Artem.Bityutskiy@nokia.com>2008-01-16 17:38:44 +0200
committerArtem Bityutskiy <Artem.Bityutskiy@nokia.com>2008-01-16 17:38:44 +0200
commitf3f219527f69d581a1839e93b749c29545eb0119 (patch)
treeb8604340671673911f6a0c94d843785081c7bdb9 /jittertest/README
parentd78b73e2ca216777e17066091ef449e9cdaffae7 (diff)
tests: move jittertest to tests directory
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
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-$Id: README,v 1.2 2001/08/10 19:23:11 vipin Exp $
-
-This is the README file for the JitterTest (and friends)
-program.
-
-This program is used to measure what the jitter of a
-real time task would be under "standard" Linux.
-
-More particularly, what is the effect of running
-a real time task under Linux with background
-JFFS file system activity.
-
-The jitter is measured in milli seconds (ms) from
-the expected time of arrival of a signal from a
-periodic timer (set by the task) to when the
-task actually gets the signal.
-
-This jitter is then stored in a file specified
-(or the default output file "jitter.dat").
-
-The data may also be sent out to the console by
-writing to /dev/console (See help options. This is
-highly desirable specially if you have redirected
-your console to the serial port and are storing it
-as a minicom log on another computer for later analysis
-using some tools provided here).
-
-This is particularly useful if you have a serial
-console and are outputting "interesting" info
-from inside some kernel task or driver.
-(or something as simple as a "df" program running
-periodically and redirecting o/p to the console).
-
-One "interesting" thing that I have measured
-is the effect of FLASH chip erases on the jitter
-of a real time task.
-
-One can do that by putting a printk at the
-beginning of the flash erase routine in the MTD
-flash chip driver.
-
-Now you will get jitter data interspersed with
-flash sector erase events. Other printk's can also
-be placed at suspected jitter causing locations in
-the system.
-
-
-
-EXECUTING THE PROGRAM "JitterTest"
-
-You may specify a file to be read by the
-program every time it wakes up (every cycle).
-This file is created and filled with some junk
-data. The purpose of this is to test the jitter
-of the program if it were reading from- say
-a JFFS (Journaling Flash File System) file system.
-
-By specifying the complete paths of the read and write
-(o/p) files you can test the jitter a POSIX type
-real time task will experience under Linux, under
-various conditions.
-
-These can be as follows:
-
-1. O/P file on ram file system, no i/p file.
-
- In this case you would presumably generate other
-"typical" background activity for your system and
-examine the worst case jitter experienced by
-a task that is neither reading nor writing to
-a file system.
-
-Other cases could be:
-
-2. O/P to ram fs, I/P from JFFS (type) fs:
-
- This is specially useful to test the proper
-operation of erase suspend type of operation
-in JFFS file systems (with an MTD layer that
-supports it).
-
- In this test you would generate some background
-write/erase type activity that would generate
-chip erases. Since this program is reading from
-the same file system, you contrast the latencies
-with those obtained with writes going to the same
-fs.
-
-3. Both read and writes to (or just write to) JFFS
-file system:
-
- Here you would test for latencies experienced by
-a program if it were writing (and optionally also
-reading) from a JFFS fs.
-
-
-
-
-Grabing a kernel profile:
-
-This program can also conditionally grab a kernel profile.
-Specify --grab_kprofile on the cmd line as well as
-a "threshold" parameter (see help options by -?).
-
-Any jitter greater than this "threshold" will cause the
-program to read the /proc/profile file and dump it in
-a local file with increasing file numbers. It will also
-output the filename at that time to the console file specified.
-This will allow you to corelate later in time the various profiles
-with data in your console file and what was going on at that time.
-
-These profile files may then be later examined by running them through
-ksymoops.
-
-Make sure you specify "profile=2" on the kernel command line
-when you boot the kernel if you want to use this functionality.
-
-
-
-Signalling the JFFS[2] GC task:
-
-You can also force this program to send a SIGSTOP/SIGCONT to the
-JFFS (or JFFS2) gc task by specifing --siggc <pid> on the cmd line.
-
-This will let you investigate the effect of forcing the gc task to
-wake up and do its thing when you are not writing to the fs and to
-force it to sleep when you want to write to the fs.
-
-These are just various tools to investigate the possibility of
-achieving minimal read/write latency when using JFFS[2].
-
-You need to manually do a "ps aux" and look up the PID of the gc
-thread and provide it to the program.
-
-
-
-
-EXECUTING THE PROGRAM "plotJittervsFill"
-
-This program is a post processing tool that will extract the jitter
-times as printed by the JitterTest program in its console log file
-as well as the data printed by the "df" command.
-
-This "df" data happens to be in the console log because you will
-run the shell file fillJffs2.sh on a console when you are doing
-your jitter test.
-
-This shell script copies a specified file to another specified file
-every programmable seconds. It also does a "df" and redirects output
-to /dev/console where it is mixed with the output from JitterTest.
-
-All this console data is stored on another computer, as all this data
-is being outputted to the serial port as you have redirected the console
-to the serial port (that is the only guaranteed way to not loose any
-console log or printk data).
-
-You can then run this saved console log through this program and it
-will output a very nice text file with the %fill in one col and
-corrosponding jitter values in the second. gnuplot then does a
-beautifull plot of this resulting file showing you graphically the
-jitters encountered at different fill % of your JFFS[2] fs.
-
-
-
-OTHER COMMENTS:
-
-Use the "-w BYTES" cmd line parameter to simulate your test case.
-Not everyone has the same requirements. Someone may want to measure
-the jitter of JFFS2 with 500 bytes being written every 500ms. Others
-may want to measure the system performance writing 2048 bytes every
-5 seconds.
-
-RUNNING MULTIPLE INSTANCES:
-
-Things get real interesting when you run multiple instances of this
-program *at the same time*.
-
-You could have one version running as a real time task (by specifing
-the priority with the -p cmd line parameter), not interacting with
-any fs or at the very least not reading and writing to JFFS[2].
-
-At the same time you could have another version running as a regular
-task (by not specifing any priority) but reading and writing to JFFS[2].
-
-This way you can easily measure the blocking performance of the real time
-task while another non-real time task interacts with JFFS[2] in the back ground.
-
-You get the idea.
-
-
-WATCH OUT!
-
-Be particularly careful of running this program as a real time task AND
-writing to JFFS[2]. Any blocks will cause your whole system to block till
-any garbage collect initiated by writes by this task complete. I have measured
-these blocks to be of the order of 40-50 seconds on a reasonably powerful
-32 bit embedded system.