Re: [dpdk-dev] Mmap query

[Brijesh Singh <brijesh.s.singh@gmail.com>]

Hi Dmitry,

Thank you for your response.  The mystery is that we are not doing mmap
anonymous, and ftrace shows high time for clear page.

I suspect, the file is being unlinked before being opened. But we haven't
been able to trace it yet.


Regards,
Brijesh


On Mon, Oct 11, 2021 at 12:38 PM Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
wrote:

> Hi Brijesh,
>
> 2021-10-11 10:25 (UTC-0700), Brijesh Singh:
> > Hi there,
> > We are seeing high mmap time for 1G hugepage during eal memalloc init.
> Most
> > time is spent for clearing page, each 4k size..
> > It's not clear to me why clear huge pages is called? The code mmaps
> rte_map
> > file with map_populate and map_shared this shouldn't cause page zeroing.
> > I am curious to know how zeroing is forced?
>
> The kernel clears all anonymous memory before handling it to the user
> space.
> It is done for security: there may be sensitive data left by previous users
> of those pages.
>
> There is a pending patchset that aims to solve your issue:
> https://mails.dpdk.org/archives/dev/2021-October/223723.html
> Please see commit messages for some more explanations.
>
> > As an alternative, I would like to memset from userspace. In my
> experiment
> > we are noticing memset 1G file is faster that clear huge pages.
>
> Can you share the numbers? From my experiments, mmap() is just a little
> longer then memset() of the same amount of memory, apparently because the
> kernel has some extra work to set up the mapping.
>