About memory coherency

[Burakov, Anatoly]

There are two different issues at play here.

The purpose of “no-huge” flag is to run DPDK without requiring hugepage memory. Originally, this has been done using an anonymous mmap() call – so, this memory was not using any fd’s at all. This presents a problem with vhost-user, because it relies on fd’s for its shared memory implementation. This is what memfd (a relatively recent addition to the kernel) is addressing – it’s enabling usage of vhost-user with no-huge because memfd actually does create an fd to back our memory.

That said, while description says “malloc”, it is technically incorrect because there’s no malloc involved in the process. The “malloc” term is simply shorthand for “use regular memory”, and should be understood in that context.

Thanks,
Anatoly

From: Kinsella, Ray <ray.kinsella at intel.com>
Sent: Tuesday, August 9, 2022 10:04 AM
To: Nick Tian <nick.tian at longsailingsemi.com>; users at dpdk.org
Cc: Burakov, Anatoly <anatoly.burakov at intel.com>
Subject: RE: About memory coherency


I may be incorrect, but is it not simply the case, that when using the no-huge parameter that MAP_HUGETLB is omitted from flags?

Ray K

From: Nick Tian <nick.tian at longsailingsemi.com<mailto:nick.tian at longsailingsemi.com>>
Sent: Tuesday 9 August 2022 03:55
To: users at dpdk.org<mailto:users at dpdk.org>
Subject: About memory coherency

Hi
I am confusing about the "no-huge" option of DPDK 21.11.
The dpdk usage said: --no-huge:Use malloc instead of hugetlbfs.
But when I check the EAL source code, I found some code piece like this:
It's look like "no-huge" option will lead dpdk use memfd_create-->ftruncate-->mmap to reserve memory
and then provide to application with rte_malloc.
Am I right?
If so, what the "malloc" in "use malloc instead of hugelbfs" refer to?

EAL_memory.c
static int eal_legacy_hugepage_init(void){
....
 if (internal_conf->no_hugetlbfs) {
....
#ifdef MEMFD_SUPPORTED
  /* create a memfd and store it in the segment fd table */
  memfd = memfd_create("nohuge", 0);
......
  /* we got an fd - now resize it */
   if (ftruncate(memfd, internal_conf->memory) < 0) {
.....
   fd = memfd;
    flags = MAP_SHARED;   }
....
  prealloc_addr = msl->base_va;
  addr = mmap(prealloc_addr, mem_sz, PROT_READ | PROT_WRITE,
    flags | MAP_FIXED, fd, 0);
...


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