From: Chuck Ebbert <76306.1226@compuserve.com>
To: linux-kernel <linux-kernel@vger.kernel.org>
Subject: Performance analysis of i386 SMP timer interrupt
Date: Sat, 4 Dec 2004 14:51:31 -0500 [thread overview]
Message-ID: <200412041453_MC3-1-900E-1EAD@compuserve.com> (raw)
Performance analysis of arch/i386/kernel/apic.c::smp_local_timer_interrupt:
{
int cpu = smp_processor_id();
profile_tick(CPU_PROFILING, regs);
1 -> if (--per_cpu(prof_counter, cpu) <= 0) {
per_cpu(prof_counter, cpu) = per_cpu(prof_multiplier, cpu);
2 -> if (per_cpu(prof_counter, cpu) !=
per_cpu(prof_old_multiplier, cpu)) {
3 -> __setup_APIC_LVTT(
calibration_result/
per_cpu(prof_counter, cpu));
4 -> per_cpu(prof_old_multiplier, cpu) =
per_cpu(prof_counter, cpu);
}
5 -> update_process_times(user_mode(regs));
}
}
(1) Needs likely().
(2) Needs unlikely().
(3,4) (4) should be done before (3) to minimize register usage.
(function becomes 6 insns smaller and uses 8 bytes less stack space)
(5) The user_mode() macro is inefficient:
#define user_mode(regs) ((VM_MASK & (regs)->eflags) || (3 & (regs)->xcs))
(VM86 mode is very rare compared to normal cpu mode, so both
parts of the 'or' are evaluated almost every time on most systems.)
--Chuck Ebbert 04-Dec-04 14:50:39
reply other threads:[~2004-12-04 19:54 UTC|newest]
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