|| ||Michael Holzheu <email@example.com> |
|| ||Shailabh Nagar <firstname.lastname@example.org>,
Andrew Morton <email@example.com>,
Venkatesh Pallipadi <firstname.lastname@example.org>,
Suresh Siddha <email@example.com>,
Peter Zijlstra <firstname.lastname@example.org>,
Ingo Molnar <email@example.com>, Oleg Nesterov <firstname.lastname@example.org>,
John stultz <email@example.com>,
Thomas Gleixner <firstname.lastname@example.org>,
Balbir Singh <email@example.com>,
Martin Schwidefsky <firstname.lastname@example.org>,
Heiko Carstens <email@example.com>,
Roland McGrath <firstname.lastname@example.org> |
|| ||[RFC][PATCH v2 0/7] taskstats: Enhancements for precise process accounting (version 2) |
|| ||Thu, 11 Nov 2010 18:03:52 +0100|
|| ||email@example.com, firstname.lastname@example.org|
|| ||Article, Thread
CHANGE HISTORY OF THIS PATCH SET
* The following patches from version 1 have been accepted upstream for 2.6.37:
+ taskstats: use real microsecond granularity for CPU times
git commit: d57af9b2142f31a39dcfdeb30776baadfc802827
+ taskstats: separate taskstats commands
git commit: 9323312592cca636d7c2580dc85fa4846efa86a2
+ taskstats: split fill_pid function
git commit: 3d9e0cf1fe007b88db55d43dfdb6839e1a029ca5
* Comment from Andrew Morton:
I replaced the /proc/taskstats ioctls with a write interface (see patch )
* Based on discussions with Oleg Nesterov and Roland McGrath:
We have identified the following problems of the current approach for getting
100% CPU time between two taskstats snapshots without using exit events:
1)Due to POSIX POSIX.1-2001, the CPU time of processes is not accounted
to the cumulative time of the parents, if the parents ignore SIGCHLD
or have set SA_NOCLDWAIT. This behaviour has the major drawback that
it is not possible to calculate all consumed CPU time of a system by
looking at the current tasks. CPU time can be lost.
2)When a parent process dies, its children get the init process as
new parent. For accounting this is suboptimal, because then init
gets the CPU time of the tasks. For accounting it would be better,
if the CPU time is passed along the relationship tree using the
cumulative time counters as would have happened if the child had died
before the parent. The main benefit of this approach is that between
two task snapshots it is always clear which parent got the CPU time
of dead tasks. Otherwise there are situations, where we can't
determine if either init or an older relative of a dead task has
inherited the CPU time.
3)If a non-leader thread calls exec(), in the de_thread() function it
gets some of the identity of the old thread group leader e.g. the PID
and the start time. But it keeps the old CPU times. This can lead to
confusion in user space, because CPU time can go backwards for the thread
As a result of this discussion I have developed a new patch  (see below):
* It adds a second set of accounting data to the signal_struct.
This set is used to save all accounting data and resolves issue (1).
* In addition to that, the patch adds a new accounting hierarchy for
processes in the signal_struct to resolve issue (2).
There are three possible alternatives to this approach:
* Introduce "reparent" events for tasks that get init as new parent.
* Get all task exit events (very expensive).
* Live with the fact that accounting without exit events cannot be done
100% accurate on UNIX/Linux and provide a solution that works with the
cumulative time accounting as it is currently available under Linux.
I have developed an additional patch  to resolve issue (3).
It swaps the accounting data between the forking thread and the thread
group leader in the de_thread() function. But perhaps another solution
should be found for that problem because this might cause problems with
the scheduler that gets confused by the swapped values.
DESCRIPTION OF PATCH SET
Currently tools like "top" gather the task information by reading procfs
files. This has several disadvantages:
* It is very CPU intensive, because a lot of system calls (readdir, open,
read, close) are necessary.
* No real task snapshot can be provided, because while the procfs files are
read the system continues running.
* It is not possible to identify 100% of all consumed CPU time between two
* The procfs times granularity is restricted to jiffies.
In parallel to procfs there is the taskstats binary interface that uses
netlink sockets as transport mechanism to deliver task information to
user space. There is a taskstats command "TASKSTATS_CMD_ATTR_PID"
to get task information for a given PID. This command can already be used for
tools like top, but has also several disadvantages:
* You first have to find out which PIDs are available in the system. Currently
we have to use procfs again to do this.
* For each task two system calls have to be issued (First send the command and
then receive the reply).
* No snapshot mechanism is available.
GOALS OF THIS PATCH SET
The intention of this patch set is to provide better support for tools like
top. The goal is to:
* provide a task snapshot mechanism where we can get a consistent view of
all running tasks.
* identify 100% of the consumed CPU time between two snapshots without
using task exit events.
* provide a transport mechanism that does not require a lot of system calls
and that allows implementing low CPU overhead task monitoring.
* provide microsecond CPU time granularity.
Together with this kernel patch set also user space code for a new top
utility (ptop) is provided that exploits the new kernel infrastructure. See
patch 6 for more details.
TEST1: System with many sleeping tasks
for ((i=0; i < 1000; i++))
sleep 1000000 &
pid user sys ste total Name
(#) (%) (%) (%) (%) (str)
541 0.37 2.39 0.10 2.87 top
3743 0.03 0.05 0.00 0.07 ptop_new_proc
Compared to the old top command that has to scan more than 1000 proc
directories the new ptop consumes much less CPU time (0.05% system time
on my s390 system).
TEST2: Show snapshot consistency with system that is 100% busy
System with 2 CPUs:
for ((i=0; i < $(cat /proc/cpuinfo | grep "^processor" | wc -l); i++))
make -j 5
pid user sys stea cuser csys cstea xuser xsys xstea total Name
(#) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (str)
2802 43.22 0.35 0.22 0.00 0.00 0.00 0.00 0.00 0.00 43.79 loop
2799 35.96 0.33 0.21 0.00 0.00 0.00 0.00 0.00 0.00 36.50 loop
2811 0.04 0.05 0.00 23.22 12.97 0.19 0.00 0.00 0.00 36.46 make
2796 35.80 0.32 0.19 0.00 0.00 0.00 0.00 0.00 0.00 36.30 loop
2987 15.93 2.14 0.07 8.23 3.12 0.06 0.00 0.00 0.00 29.53 make
3044 11.56 1.72 0.22 0.00 0.00 0.00 0.00 0.00 0.00 13.50 make
3053 1.92 0.73 0.01 0.00 0.00 0.00 0.00 0.00 0.00 2.65 make
V:V:S 144.76 6.24 1.22 31.44 16.09 0.25 0.00 0.00 0.00 200.00
With the snapshot mechanism the sum of all tasks CPU times will be exactly
200.00% CPU time with this testcase. The following CPU times are used:
* user/sys/stea: Time that has been consumed by task itself
* cuser/csys/cstea: Time that has been consumed by dead children of task
* xuser/xsys/xstea: Time that has been consumed by dead threads of thread
group of task
Note that the CPU times on x86 are not as precise as on s390.
Patches apply on:
The code is not final and still has several TODOs. The following kernel patches
 Add new command "TASKSTATS_CMD_ATTR_PIDS" to get a snapshot of multiple
 Add procfs interface for taskstats commands. This allows to get a complete
and consistent snapshot with all tasks using two system calls (write and
read). Transferring a snapshot of all running tasks is not possible using
the existing netlink interface, because there we have the socket buffer
size as restricting factor.
 Add TGID to taskstats.
 Add steal time per task accounting.
 Improve cumulative CPU time accounting.
 Fix accounting for non-leader thread exec.
 Besides of the kernel patches also user space code is provided that
exploits the new kernel infrastructure. The user space code provides the
1. A proposal for a taskstats user space library:
1.1 Based on netlink (requires libnl-devel-1.1-5)
2.1 Based on the new /proc/taskstats interface (see patch )
2. A proposal for a task snapshot library based on taskstats library (1.1)
3. A new tool "ptop" (precise top) that uses the libraries
* Especially patch  "Add new command "TASKSTATS_CMD_ATTR_PIDS" to
get a snapshot of multiple tasks" needs more review because it provides
the main functionality for getting consistent task snapshots. Up to now
I did not get any feedback on version 1 of this patch.
* Also the user space part with the libraries  would need some review.
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