Re: strange DPDK memory allocator behavior

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

On 2024-11-08 17:08, Mattias Rönnblom wrote:
> On 2024-11-08 16:03, Thomas Monjalon wrote:
>> Hello,
>>
>> After merging the lcore variables allocation from Mattias,
>> we've seen an issue in unit tests running on 32-bit plaftorms:
>>     https://build.opensuse.org/package/live_build_log/home:bluca:dpdk/ 
>> dpdk/Debian_Testing/i586
>> The initialization has this error message "most of the time":
>>     EAL: Cannot get a virtual area: Cannot allocate memory
>>
>> David did more tests with this command:
>> # (for i in $(seq 10); do echo | LD_LIBRARY_PATH=lib:drivers app/dpdk- 
>> test --no-huge -m 2048 -d drivers; done) |& grep 'Cannot get a virtual 
>> area: Cannot allocate memory'
>>
>> EAL: Cannot get a virtual area: Cannot allocate memory
>> EAL: Cannot get a virtual area: Cannot allocate memory
>> EAL: Cannot get a virtual area: Cannot allocate memory
>>
>> There are 2 interesting findings:
>>
>> - the issue disappears when changing allocation size 2048
>>    to more (4096) or less (1024)
>>
>> - the issue disappears when removing some big static arrays from unit 
>> tests
>>    like "sarray_lcore_state" in test_lcore_var_perf.c
>>    Should we avoid having such big arrays in our unit tests?
> 
> Wouldn't that be just removing a test that triggers a DPDK bug, rather 
> than fixing the bug? 128 kB is not big and some other app could easily 
> have such objects in BSS.
> 

128 kB should be 16 MB. What is a factor of 128 between friends?

>>
>> I don't understand how "-m 2048" is impacted by the static allocations.
> 
> The ~128 kB of BSS will change the addresses of the heap to generally 
> somewhat higher numbers, since the heap is mapped in after BSS (I 
> think). But with ASLR, I'm not sure it would matter. I'm also not sure 
> how ASLR works on 32-bit systems, so maybe it could matter.
> 
> Are there any attempts made to mmap() something at some particular 
> hardcoded address? Far fetched.
> 
>> And I wonder how it could impact memory fragmentation
>> so 2048 MB cannot be allocated but more or less are working fine.
>>
> 
> If you get an error during EAL initialization, pages to host those 
> test_lcore_var_perf static arrays aren't allocated yet.
> 
>> It looks like an issue in the DPDK allocator.
>> Note that we are using --no-huge for these tests.
>> Any ideas?
>>
>>
>