Re: [dpdk-dev] [PATCH v5] eal: use memzone to share tsc hz with secondary processes

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Aug 27, 2019 at 01:04:18PM +0100, Burakov, Anatoly wrote:
> On 26-Aug-19 2:44 PM, Jim Harris wrote:
> > Ideally, get_tsc_freq_arch() is able to provide the
> > TSC rate using arch-specific means.  When that is not
> > possible, DPDK reverts to calculating the TSC rate with
> > a 100ms nanosleep or 1s sleep.  The latter occurs more
> > frequently in VMs which often do not have access to the
> > data they need from arch-specific means (CPUID leaf 0x15
> > or MSR 0xCE on x86).
> > 
> > In secondary processes, the extra 100ms is especially
> > noticeable and consumes the bulk of rte_eal_init()
> > execution time.  To resolve this extra delay, have
> > the primary process put the TSC rate into a shared
> > memory region that the secondary process can lookup.
> > 
> > Reduces rte_eal_init() execution time in a secondary
> > process from 165ms to 66ms on my test system.
> > 
> > Signed-off-by: Jim Harris <james.r.harris@intel.com>
> > ---
> 
> I think this is a bad idea. If you're allocating something, you're supposed
> to free it in rte_eal_cleanup(). If you don't, that memory leaks (i.e. there
> are leftover hugepages after process exit). Since both primary and secondary
> are referencing it (even if only at init), there is no safe way to free this
> memzone.
> 
Actually, after looking at the code again, the secondary processes only
reference the memzone once on init, and fallback to the current path if the
memzone does not exist. Therefore, it should be safe to remove the memzone
on termination of the primary process.