Re: User cpu time VS system cpu time
Hello, I want to understand difference between user CPU time and system CPU time in system accounting. But keep in mind that kernel time is a broad category - while IO time in itself does not count as CPU time, file system operations for example do, because they really can be CPU intensive. Ivan, thanks for the great explanation. I think that I can measure user filesystem usage with sa - it reports number of IO operations per user/command. In which other cases kernel time is used instead of user time for a process? I do not mean all of them - just that usually occur in practice. I've noticed that there are moments when system load in top for system time is very high (60-80% while user load is 15-25%, this produces very high LA also). All processes that were run at this time show high kernel time usage, although they usually do not. System is getting back to normal after Apache restart (I think this is related to Apache shared memory somehow, but not sure). This makes me suspect that system time in sa can not be relied on while measuring user system usage, because it notably varies under some circumstances for same operations. Am I wrong? -- // cronfy ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
Re: User cpu time VS system cpu time
On 05/06/10 13:33, cronfy wrote: Hello, I want to understand difference between user CPU time and system CPU time in system accounting. But keep in mind that kernel time is a broad category - while IO time in itself does not count as CPU time, file system operations for example do, because they really can be CPU intensive. Ivan, thanks for the great explanation. I think that I can measure user filesystem usage with sa - it reports number of IO operations per user/command. In which other cases kernel time is used instead of user time for a process? I do not mean all of them - just that usually occur in practice. Everything the kernel does when requested by the user is counted as kernel time - file system access, network access, getpid(), gettimeofday(), process scheduling, memory management, etc. I've noticed that there are moments when system load in top for system time is very high (60-80% while user load is 15-25%, this produces very high LA also). All processes that were run at this time show high kernel time usage, although they usually do not. System is getting back to normal after Apache restart (I think this is related to Apache shared memory somehow, but not sure). As I told you before - monitor the top line for the processes you suspect and you will get a fairly good idea what they are doing. Look at the STATE column. When you are looking at per-process statistics, the system time is also accounted. For example, if a process takes 50% of a CPU, it is possible that it takes 25% in userspace and 25% in kernel (the reverse is not true - kernel can take system CPU time without it being accounted on behalf of processes). This makes me suspect that system time in sa can not be relied on while measuring user system usage, because it notably varies under some circumstances for same operations. Am I wrong? Everything can be accounted for by enough statistics :) ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
Re: User cpu time VS system cpu time
From: cronfy cro...@gmail.com Date: Thu, 6 May 2010 15:33:40 +0400 Subject: Re: User cpu time VS system cpu time Hello, I want to understand difference between user CPU time and system CPU time in system accounting. But keep in mind that kernel time is a broad category - while IO time in itself does not count as CPU time, file system operations for example do, because they really can be CPU intensive. Ivan, thanks for the great explanation. I think that I can measure user filesystem usage with sa - it reports number of IO operations per user/command. In which other cases kernel time is used instead of user time for a process? I do not mean all of them - just that usually occur in practice. I've noticed that there are moments when system load in top for system time is very high (60-80% while user load is 15-25%, this produces very high LA also). All processes that were run at this time show high kernel time usage, although they usually do not. System is getting back to normal after Apache restart (I think this is related to Apache shared memory somehow, but not sure). This makes me suspect that system time in sa can not be relied on while measuring user system usage, because it notably varies under some circumstances for same operations. Am I wrong? CPU time tracking is -really- simple to understand. logically you look at where the PC is, at regular intervals. it is in one of 3 types of locations -- in 'user' space, somewhere 'inside' the kernel itself, *OR* in the system 'idle loop'. Time spent executing _most_ kernel functions (system calls) is _not_ strictly deterministic -- it depends on what all 'else' the O/S is doing at the time, as well as that which is 'strictly necessary' to perform just the user-initiated action. take a simple case of appending a block of data to a disk file (Berkeley FFS). Assume the file pointer is already at EOF -- it may be that the data fits into the unused part of the last already allocated block for the file, or it *MAY*NOT*. If not there is extra work to do -- get a block from the free list, zero it, copy the user data into it, and add the block to the block-list for that file. It may be possible to record that block's address in the space already allocated for the block list, or it MAY NOT. If not, one has to get a block from the free list, and add it to the 'meta-data' for the file. This _may_ necessitate adding a 2nd-level index block, which *MAY* necessitate adding a 3rd level index block, (and possibly a 4th). Adding to the 'uncertainty' of the numbers, _between_ sampling intervals it is possible for an interrupt to occur, be serviced, and control returned to the lower priority code. If the interrupt-service duration is _less_ than the sampling interval, then the interrupt-service time gets counted, as if it were part of the 'class' of code that was interrupted. This can result in small amounts of what should be 'system' time for one user getting charged as time (user -or- system) for a different user. Similar things can happen when transferring data to/from other kinds of devices, e.g. printers, terminals, etc. On a busy system, there can be a variance of 20% or more, between two successive runs of the same job. ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
Re: User cpu time VS system cpu time
On 05/03/10 18:00, cronfy wrote: Hello, I want to understand difference between user CPU time and system CPU time in system accounting. When some process uses many system CPU, does it really mean that process prouduces heavy load on server and takes up resources that could be used by other tasks instead? Or it only means that this process performs many waits for, say, I/O operations? In contrast to Linux (luckily), IO wait in itself in FreeBSD does not count as CPU time. Linux's iowait state is supposedly defined as idle but having outstanding IO requests but judging from performance curves in some cases I think some polling is involved. System time is time spent in the kernel - it is practically always the result of issuing kernel syscalls from userland processes. Whether it influences the overall performance of the system depends on what the processes are doing - if the machine has multiple CPU cores and the syscalls exercise those parts of the system that are parallelized then no, your process would not influence the rest of the system anymore than it would by using user time. But keep in mind that kernel time is a broad category - while IO time in itself does not count as CPU time, file system operations for example do, because they really can be CPU intensive. And by using the file system in certain ways, you really can block other processes from using it with the performance they would get otherwise. You can find out at a glance approximately what the process is doing simply by monitoring the top line for the process. There is one more thing - if the machine in question is a virtual machine, it is normal for the system time to, unfortunately, be significantly higher than it would be otherwise because of the nature of the emulation. ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
User cpu time VS system cpu time
Hello, I want to understand difference between user CPU time and system CPU time in system accounting. When some process uses many system CPU, does it really mean that process prouduces heavy load on server and takes up resources that could be used by other tasks instead? Or it only means that this process performs many waits for, say, I/O operations? Thanks in advance! -- // cronfy ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
