Re: [dpdk-dev] [PATCH v2 2/5] mem: use address hint for mapping hugepages

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

On 04-Oct-18 12:43 PM, Alejandro Lucero wrote:
> 
> 
> On Wed, Oct 3, 2018 at 1:50 PM Burakov, Anatoly 
> <anatoly.burakov@intel.com <mailto:anatoly.burakov@intel.com>> wrote:
> 
>     On 31-Aug-18 1:50 PM, Alejandro Lucero wrote:
>      > Linux kernel uses a really high address as starting address for
>      > serving mmaps calls. If there exist addressing limitations and
>      > IOVA mode is VA, this starting address is likely too high for
>      > those devices. However, it is possible to use a lower address in
>      > the process virtual address space as with 64 bits there is a lot
>      > of available space.
>      >
>      > This patch adds an address hint as starting address for 64 bits
>      > systems.
>      >
>      > Signed-off-by: Alejandro Lucero <alejandro.lucero@netronome.com
>     <mailto:alejandro.lucero@netronome.com>>
>      > ---
> 
>     <snip>
> 
>      >
>      >               mapped_addr = mmap(requested_addr, (size_t)map_sz,
>     PROT_READ,
>      >                               mmap_flags, -1, 0);
>      > +
>      >               if (mapped_addr == MAP_FAILED && allow_shrink)
> 
>     Unintended whitespace change?
> 
> 
> Yes. I'll fix it.
> 
>      >                       *size -= page_sz;
>      > -     } while (allow_shrink && mapped_addr == MAP_FAILED && *size
>      > 0);
>      > +
>      > +             if (mapped_addr != MAP_FAILED && addr_is_hint &&
>      > +                 mapped_addr != requested_addr) {
>      > +                     /* hint was not used. Try with another
>     offset */
>      > +                     munmap(mapped_addr, map_sz);
>      > +                     mapped_addr = MAP_FAILED;
>      > +                     next_baseaddr = RTE_PTR_ADD(next_baseaddr,
>     0x100000000);
> 
>     Why not increment by page size? Sure, it could take some more time to
>     allocate, but will result in less wasted memory.
> 
> 
> I though the same or even using smaller increments than hugepage size. 
> Increment the address in such amount does not mean we are wasting memory 
> but just leaving space if some mmap fails. I think it is better to leave 
> as much as space as possible just in case the data allocated in the 
> conflicted area would need to grow in the future.

Not sure i follow. Could you give an example of a scenario where leaving 
huge chunks of memory free would be preferable to just adding page size 
and starting from page-size-aligned address next time we allocate?

> 
>     -- 
>     Thanks,
>     Anatoly
> 


-- 
Thanks,
Anatoly