Date: Thu, 24 Mar 2011 17:00:02 +0800 From: Jing Huang <jing.huang.pku@gmail.com> To: kris@freebsd.org Cc: freebsd-hackers@freebsd.org, hackers@FreeBSD.org Subject: Timecounter Project (GSoc2011) Message-ID: <AANLkTim1CHsasQTNqGDSHqP_Vesvb03FxDixiLfEJu8_@mail.gmail.com>
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Hi Everyone,
I am a student of Peking University in China. I am interest
in the FreeBSD project of "Timecounter Performance Improvements".
I am familiar with Linux kernel and virtualization systems,
like KVM and Xen. I have maintained the Linux Server for my College
for last whole year. Recently, I learned a lot about KVM and assigned
VMs to students who need them. I also have experience of install and
config FreeBSD system.
In this scenario, I plan to use both tsc and shared memory to
calculate precise time in user mode. The shared memory includes
system_time, tsc_system_time and factor_tsc-system_time.
a) system_time is the current time since the system boots, and it will
be updated by time interrupt handling function.
b) tsc_system_time is the tsc value when system_time update, and it is
used to calculate rough delta.
c) factor_tsc-system_time records the correspondence between tsc and
time. It is used to calculate precise delta.
When the process invokes gettimeofday, our mechanism will
calculate time like: system_time+ (
factor_tsc-system_time(instant_rdtsc - tsc_system_time ) ). We see
factor_tsc-system_time as a computing method here. I could refer to
the relative implementation in KVM or Xen virtual machine.
The real problem is that tsc counter is CPU sensitive. Our
process will get wrong tsc value when switching to another CPU. I plan
to force the FreeBSD kernel to update shared memory when monitoring
context switch event. Alternatively, we can build NR_CPUS shared
memories, and every CPU has one. Hmm.. how to recognize which CPU we
are using in user mode?
We also consider the CPU frequency, because tsc counter is
related to it. When kernel changes CPU frequency, the shared memory
should be update subsequently.
Would some one give me some suggestions? Looking forward to your reply.
yours sincerely.
Jing
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