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

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

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?

> 
>     -- 
>     Thanks,
>     Anatoly
> 


-- 
Thanks,
Anatoly