Re: [PATCH] config: limit lcore variable maximum size to 4k

["Mattias Rönnblom" <hofors@lysator.liu.se>]

On 2024-11-09 00:11, Thomas Monjalon wrote:
> 08/11/2024 23:34, Mattias Rönnblom:
>> On 2024-11-08 23:13, Thomas Monjalon wrote:
>>> 08/11/2024 20:53, Morten Brørup:
>>>>> From: Morten Brørup [mailto:mb@smartsharesystems.com]
>>>>> Sent: Friday, 8 November 2024 19.35
>>>>>
>>>>>> From: David Marchand [mailto:david.marchand@redhat.com]
>>>>>> Sent: Friday, 8 November 2024 19.18
>>>>>>
>>>>>> OVS locks all pages to avoid page faults while processing packets.
>>>>
>>>> It sounds smart, so I just took a look at how it does this. I'm not sure, but it seems like it only locks pages that are actually mapped (current and future).
>>>>
>>>>>> 1M for each lcore translates to allocating 128M with default build
>>>>>> options on x86.
>>>>>> This resulted in OOM while running unit tests in parallel.
>>>>
>>>> Is the root cause the lcore variables library itself, or the unit test using a lot of memory for testing the lcore variables?
>>>> We don't want to fix the library if the problem is elsewhere.
>>>
>>> The fix works for our urgent issue and we want to make a release candidate soon.
>>>
>>>
>>>>>> At the moment, the more demanding DPDK user of lcore variable is
>>>>>> rte_service, with a 2112 bytes object.
>>>>>>
>>>>>> Limit the lcore variable maximum size to 4k which looks more
>>>>>> reasonable.
>>>>>
>>>>> 4 KB is not future proof.
>>>>>
>>>>> Here's an example where 16 KB is cutting it close:
>>>>> https://inbox.dpdk.org/dev/98CBD80474FA8B44BF855DF32C47DC35E9F7D0@smart
>>>>> server.smartshare.dk/
>>>>>
>>>>> Depends on how we are going to use it. 4 KB suffices if we only want to
>>>>> use it for "small" structures.
>>>
>>> This is what is stated in the doc:
>>> "Lcore variables are suitable for small objects"
>>> "The amount of data kept in lcore variables is projected to be small"
>>>>>> Would 64 KB work as a compromise?
>>>
>>> Let's consider based on the need.
>>> The lcore variables are new and we don't want it to degrade the DPDK footprint,
>>> at least not in this first version.
>>> 4 KB is a memory page on common systems,
>>> it looks reasonnable and big enough for a "variable".
>>>
>>> Applied, thanks.
>>
>> Why do you have maintainers if you just ignore them?
> 
> I didn't receive your replies when I started to write this.
> Please be comprehensive.
> We are in a hurry to stabilize this before the release candidate which is already late.
> 
> I'll change to 128 KB as you recommend before pushing to the repository.
> 

Thanks.

> PS: maybe I should not have merged this feature in 24.11.
> 

Ideally, it should have been merged earlier in the release cycle. I did 
offer the documentation you asked for as a separate patch, to be 
delivered later.