Re: [dpdk-dev] [PATCH 17.11] mem: fix memory initialization time

[Alejandro Lucero <alejandro.lucero@netronome.com>]

On Fri, Nov 16, 2018 at 1:35 PM Burakov, Anatoly <anatoly.burakov@intel.com>
wrote:

> On 16-Nov-18 12:49 PM, Alejandro Lucero wrote:
> >
> >
> > On Thu, Nov 15, 2018 at 1:16 PM Burakov, Anatoly
> > <anatoly.burakov@intel.com <mailto:anatoly.burakov@intel.com>> wrote:
> >
> >     On 12-Nov-18 11:18 AM, Alejandro Lucero wrote:
> >      > When using large amount of hugepage based memory, doing all the
> >      > hugepages mapping can take quite significant time.
> >      >
> >      > The problem is hugepages being initially mmaped to virtual
> addresses
> >      > which will be tried later for the final hugepage mmaping. This
> causes
> >      > the final mapping requiring calling mmap with another hint
> >     address which
> >      > can happen several times, depending on the amount of memory to
> >     mmap, and
> >      > which each mmmap taking more than a second.
> >      >
> >      > This patch changes the hint for the initial hugepage mmaping using
> >      > a starting address which will not collide with the final mmaping.
> >      >
> >      > Fixes: 293c0c4b957f ("mem: use address hint for mapping
> hugepages")
> >      >
> >      > Signed-off-by: Alejandro Lucero <alejandro.lucero@netronome.com
> >     <mailto:alejandro.lucero@netronome.com>>
> >      > ---
> >
> >     Hi Alejandro,
> >
> >     I'm not sure i understand the purpose. When final mapping is
> performed,
> >     we reserve new memory area, and map pages into it. (i don't quite
> >     understand why we unmap the area before mapping pages, but it's how
> >     it's
> >     always been and i didn't change it in the legacy code)
> >
> >     Which addresses are causing the collision?
> >
> >
> > Because the hint for the final mapping is at 4GB address, and the
> > hugepages are initially individually mapped starting at low virtual
> > addresses, when the memory to map is 4GB or higher, the hugepages will
> > end using that hint address and higher. The more the hugepages to mmap,
> > the more addresses above the hint address are used, and the more mmaps
> > failed for getting the virtual addresses for the final mmap.
>
> Yes, but i still don't understand what the problem is.
>
> Before the final mapping, all of the pages get unmapped. They no longer
> occupy any VA space at all. Then, we create a VA-area the size of
> IOVA-contiguous chunk we have, but then we also unmap *that* (again, no
> idea why we actually do that, but that's how it works). So, the final
> mapping is performed with the knowledge that there are no pages at
> specified addresses, and mapping for specified addresses is performed
> when the first mapping has already been unmapped.
>
> As far as i understand, at no point do we hold addresses for initial and
> final mappings concurrently. So, where does the conflict come in?
>
>
Are you sure about this? Because I can see calling unmap_all_hugepage_init
happens after the second call to map_all_hugepages.

Maybe you are looking at the legacy code in a newer version which is not
exactly doing the same steps.


> >
> >     --
> >     Thanks,
> >     Anatoly
> >
>
>
> --
> Thanks,
> Anatoly
>