When jiffy count is incremented then a process gets time to run. The jiffies are incremented by the timer interrupt that tells the scheduler to reschedule. The jiffy defines the maximum time period for the processes to run without rescheduling. If the process calls
sleep(), for example, then rescheduling takes place immediately. Thus the context switch to a next available running process not necessarily occurs at the jiffy boundary.
However, the actual kernel behavior is defined at kernel compile time, linux/kernel/Kconfig.preempt:
prompt "Preemption Model"
bool "No Forced Preemption (Server)"
This is the traditional Linux preemption model, geared towards
throughput. It will still provide good latencies most of the
time, but there are no guarantees and occasional longer delays
Select this option if you are building a kernel for a server or
scientific/computation system, or if you want to maximize the
raw processing power of the kernel, irrespective of scheduling
bool "Voluntary Kernel Preemption (Desktop)"
This option reduces the latency of the kernel by adding more
"explicit preemption points" to the kernel code. These new
preemption points have been selected to reduce the maximum
latency of rescheduling, providing faster application reactions,
at the cost of slightly lower throughput.
This allows reaction to interactive events by allowing a
low priority process to voluntarily preempt itself even if it
is in kernel mode executing a system call. This allows
applications to run more 'smoothly' even when the system is
Select this if you are building a kernel for a desktop system.
bool "Preemptible Kernel (Low-Latency Desktop)"
select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
This option reduces the latency of the kernel by making
all kernel code (that is not executing in a critical section)
preemptible. This allows reaction to interactive events by
permitting a low priority process to be preempted involuntarily
even if it is in kernel mode executing a system call and would
otherwise not be about to reach a natural preemption point.
This allows applications to run more 'smoothly' even when the
system is under load, at the cost of slightly lower throughput
and a slight runtime overhead to kernel code.
Select this if you are building a kernel for a desktop or
embedded system with latency requirements in the milliseconds
Yes, Linux can't tell how many operations have been executed in 1 jiffy as different instructions takes different times to execute as well as pipelining of the instructions affects the number of instructions executed in a unit of time.