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

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 27-Aug-19 1:48 PM, Bruce Richardson wrote:
> 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.
>>
> What is the issue with not freeing it? How do we handle this for other
> global data to be shared?
> 

I don't think we have any shared data that persists after process exit. 
Or at least not in EAL we don't - i'm aware that some drivers leak 
memory in that way, but that's on them, not on me :) The immediate 
consequence of such an approach is failed EAL flags unit tests (because 
they expect that nothing is left after DPDK process termination).

Most shared data in libraries is explicitly initialized/deinitialized by 
the library user, so it's not an issue. An exception to that is the 
timer library, which uses refcounting to initialize/deinitialize shared 
state.

-- 
Thanks,
Anatoly