[LITMUS^RT] The gap between the release time and the real beginning of execution

[shijunjie92]

Hello:

The previous pic 'temp.jpg' is snipped from the tutor16's slides http://www.litmus-rt.org/tutor16/slides.pdf
The 'temp_2.jpg' and 'temp_3.jpg' are snipped from my platform. You can see the gaps in different period are variant and unpredictable.
I run the xubuntu img on virtualbox, and the hardware information is listed as follows:
H/W path        Device      Class       Description
===================================================
                            system      VirtualBox ()
/0                          bus         VirtualBox
/0/0                        memory      128KiB BIOS
/0/1                        memory      3952MiB System memory
/0/2                        processor   Intel(R) Core(TM) i5-3317U CPU @ 1.70GHz
/0/100                      bridge      440FX - 82441FX PMC [Natoma]
/0/100/1                    bridge      82371SB PIIX3 ISA [Natoma/Triton II]
/0/100/1.1                  storage     82371AB/EB/MB PIIX4 IDE
/0/100/2                    display     VirtualBox Graphics Adapter
/0/100/3        eth0        network     82540EM Gigabit Ethernet Controller
/0/100/4                    generic     VirtualBox Guest Service
/0/100/5                    multimedia  82801AA AC'97 Audio Controller
/0/100/6                    bus         KeyLargo/Intrepid USB
/0/100/7                    bridge      82371AB/EB/MB PIIX4 ACPI
/0/100/d                    storage     82801HM/HEM (ICH8M/ICH8M-E) SATA Control
/0/3            scsi1       storage     
/0/3/0.0.0      /dev/cdrom  disk        DVD reader
/0/4            scsi2       storage     
/0/4/0.0.0      /dev/sda    disk        34GB VBOX HARDDISK
/0/4/0.0.0/1    /dev/sda1   volume      29GiB EXT4 volume
/0/4/0.0.0/2    /dev/sda2   volume      2099MiB Extended partition
/0/4/0.0.0/2/5  /dev/sda5   volume      2097MiB Linux swap / Solaris partition

2016-07-19 

shijunjie92 



发件人：Björn Brandenburg <bbb at mpi-sws.org>
发送时间：2016-07-19 10:43
主题：Re: [LITMUS^RT] The gap between the release time and the real beginning of execution
收件人："litmus-dev"<litmus-dev at lists.litmus-rt.org>
抄送：



On 19 Jul 2016, at 11:13, shijunjie92 <shijunjie92 at gmail.com> wrote:


When I use the feature trace tool to draw the Gantt chart of the cpus' execution. I notice that there is a gap between the release time and the beginning of the real execution. I have traced the code of 'rtspin.c' but found nothing. Can anyone explain the reason of the random gap or how to reduce the gap? (sometimes the gap is 2 ms)


That’s release latency. Either your platform’s timers are extremely imprecise (this is usually the case under virtualization), or you have extremely long non-preemptive sections, or there is some extreme overhead. In mainline LITMUS^RT (with a normal config), I would rule out the last two issues.


What hardware platform are you running your test on? This looks like a trace from a VM to me.


- Björn
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