Re: [dpdk-dev] DPDK hugepage memory fragmentation

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

On 16-Sep-20 12:19 PM, Burakov, Anatoly wrote:
> On 16-Sep-20 5:32 AM, Kamaraj P wrote:
>> Hi Anatoly,
>>
>> We just dump the memory contents when it fails to allocate the memory.
>>
> 
> Hi Kamaraj,
> 
> Yes, i can see that the memory is fragmented. That's not what i was 
> asking though, because memory fragmentation is *expected* if you're 
> using igb_uio. You're not using VFIO and IOVA as VA addressing, so 
> you're at the mercy of your kernel when it comes to getting 
> IOVA-contiguous address. We in DPDK cannot do anything about it, because 
> we don't control which pages we get and what addresses they get 
> assigned. There's nothing to fix on our side in this situation.
> 
> Here are the things you can do to avoid allocation failure in your case:
> 
> 1) Drop the IOVA-contiguous allocation flag [1]
> 2) Use legacy mode [2] [3]
> 3) Switch to using VFIO [4]
> 4) Use bigger page size
> 
> The first point is crucial! Your allocation *wouldn't have failed* if 
> your code didn't specify for the allocation to require 
> IOVA-contiguousness. The fact that it has failed means that your 
> allocation has requested such memory, so it's on you to ensure that 
> whatever you're allocating, IOVA-contiguousness is *required*. I cannot 
> decide that for you as it is your code, so it is up to you to figure out 
> if whatever you're allocating actually requires such memory, or if you 
> can safely remove this allocation flag from your code.
> 
> [1] 
> http://doc.dpdk.org/api/rte__memzone_8h.html#a3ccbea77ccab608c6e683817a3eb170f 
> 
> [2] 
> http://doc.dpdk.org/guides/prog_guide/env_abstraction_layer.html#memory-mapping-discovery-and-memory-reservation 
> 
> [3] http://doc.dpdk.org/guides/linux_gsg/linux_eal_parameters.html#id3
> [4] I understand the requirement for PF driver, but i think support for 
> PF in VFIO is coming in 20.11 release
> 

Also, to add to this, we still haven't established that the allocation 
error is in fact due to memory fragmentation. You *claim* that is the 
case, but memory allocations can fail for other reasons, and you haven't 
provided me with anything that would confirm this, as *merely* the fact 
that the memory is fragmented will not, in and of itself, necessarily 
cause any allocation failures *unless* you are trying to allocate memory 
area that is bigger than your page size *and* is required to be contiguous.

If you can't tell me *what* you're allocating, at least tell me the size 
of the allocation, the memzone flags it is allocated with, and any debug 
log messages around the allocation, so that we can at least be sure that 
we know the reason why the allocation fails. You're forcing me to fly 
blind here, i can't help you if i don't know what i'm dealing with.

-- 
Thanks,
Anatoly