avner avner - 1 year ago 225
Linux Question

rdtsc accuracy across CPU cores

I am sending network packets from one thread and receiving replies on a 2nd thread that runs on a different CPU core. My process measures the time between send & receive of each packet (similar to ping). I am using rdtsc for getting high-resolution, low-overhead timing, which is needed by my implementation.

All measurments looks reliable. Still, I am worried about rdtsc accuracy across cores, since I've been reading some texts which implied that tsc is not synced between cores.

I found the following info about TSC in wikipedia

Constant TSC behavior ensures that the duration of each clock tick is
uniform and supports the use of the
TSC as a wall clock timer even if the
processor core changes frequency
. This
is the architectural behavior moving
forward for all Intel processors.

Still I am worried about accruracy across cores, and this is my question

More Info

  • I run my process on an Intel nehalem machine.

  • Operating System is Linux.

  • The "constant_tsc" cpu flag is set for all the cores.

Answer Source

X86_FEATURE_CONSTANT_TSC + X86_FEATURE_NONSTOP_TSC bits in cpuid (edx=x80000007, bit #8; check unsynchronized_tsc function of linux kernel for more checks)

Intel's Designer's vol3b, section 16.11.1 Invariant TSC it says the following

"16.11.1 Invariant TSC

The time stamp counter in newer processors may support an enhancement, referred to as invariant TSC. Processor's support for invariant TSC is indicated by CPUID.80000007H:EDX[8].

The invariant TSC will run at a constant rate in all ACPI P-, C-. and T-states. This is the architectural behavior moving forward. On processors with invariant TSC support, the OS may use the TSC for wall clock timer services (instead of ACPI or HPET timers). TSC reads are much more efficient and do not incur the overhead associated with a ring transition or access to a platform resource."

So, if TSC can be used for wallclock, they are guaranteed to be in sync.

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