Re: [dpdk-dev] Zeroing out memory on free

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

On 25-Aug-21 8:26 AM, Thomas Monjalon wrote:
> 24/08/2021 12:58, Dmitry Kozlyuk:
>> Hello,
>>
>> Me and Xueming are wondering why DPDK clears the memory on free
>> and not only when it's explicitly requested (rte_zmalloc).
>>
>> It's been so for a while:
>>
>> commit ea0bddbd14e68fb42d9774bc3543e51b510e48d3
>> Author: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>
>> Date:   Tue Jul 5 12:01:15 2016 +0100
>>
>>      mem: zero out memory on free
>>      
>>      Since commit fafcc11985a2, memzones are not guaranteed to be zeroed out.
>>      This could potentially cause issues as applications might have been
>>      relying on the allocated memory being zeroed out.
>>      
>>      On init all allocated memory is zeroed by the kernel, so by zeroing out
>>      memory on free, all available dpdk memory is always zeroed.
>>      
>>      Fixes: fafcc11985a2 ("mem: rework memzone to be allocated by malloc")
>>      
>>      Signed-off-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>
>>
>> At present this explanation doesn't look satisfying:
>> 1. Memzone API does not promise they will be zeroed out.
>>     Memzones are mostly used for DMA, so their content will likely be overwritten.
>> 2. If application assumptions are wrong, DPDK should not try to fix it.
>>     Memory manager poisons memory in debug mode to detect such errors.
>>
>> Zeroing memory is quite slow, but in many cases unneeded.
>> It looks attractive to only do it in rte_zmalloc().
>> AFAIK what prohibits moving memset() there unconditionally
>> is that the kernel already gives us cleared pages,
>> so the first allocation of each piece of memory would do unnecessary work.
>> This can be solved by giving the user a choice in EAL options
> 
> No I don't think it should be workarounded in EAL options.
> 
>> or with more elaborate tracking of memory dirtiness in MM.
> 
> Looks a good idea.
> 
>> Are there any other reasons why clearing is done the current way?
> 
> I think the only reason was to avoid doing reset for the first usage.
> You're right it is not optimal when reusing memory
> without any zeroing need.
> 

You said it yourself: zeroing out memory is quite slow. This was pretty 
much done for performance reasons - memory is released way less often 
than allocated, and we get memory from the kernel that is *always* 
zeroed out in case of hugepages, so we don't need to zero them out 
initially. So, in essence, this allows us to always keeps memory zeroed 
out without worrying about doing that for every allocation.

-- 
Thanks,
Anatoly