Re: [dpdk-dev] [PATCH] app/testpmd: adds mlockall() to fix pages

[Thomas Monjalon <thomas@monjalon.net>]

12/09/2017 22:29, Aaron Conole:
> Thomas Monjalon <thomas@monjalon.net> writes:
> 
> > 12/09/2017 16:50, Aaron Conole:
> >> Eelco Chaudron <echaudro@redhat.com> writes:
> >> 
> >> > Call the mlockall() function, to attempt to lock all of its process
> >> > memory into physical RAM, and preventing the kernel from paging any
> >> > of its memory to disk.
> >> >
> >> > When using testpmd for performance testing, depending on the code path
> >> > taken, we see a couple of page faults in a row. These faults effect
> >> > the overall drop-rate of testpmd. On Linux the mlockall() call will
> >> > prefault all the pages of testpmd (and the DPDK libraries if linked
> >> > dynamically), even without LD_BIND_NOW.
> >> >
> >> > Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
> >> 
> >> Acked-by: Aaron Conole <aconole@redhat.com>
> >
> > It is interesting, but why make it in testpmd?
> >
> > Maybe it should be documented in this guide:
> > 	http://dpdk.org/doc/guides/linux_gsg/nic_perf_intel_platform.html
> 
> Well, I'm not sure what the user would be able to do to get the
> prefaulting performance without having a library they use with
> LD_PRELOAD and a function with the constructor attribute which does the
> same thing, AND export LD_BIND_NOW before linking starts.
> 
> The LD_BIND_NOW simply does the symbol resolution, but there's no
> guarantee that it will fault all the code pages in to process space, and
> without an mlockall(), I'm not sure that there's any kind of guarantee
> that they don't get swapped out of resident memory (which also leads to
> later page faults).
> 
> Maybe I misunderstood the question?

Maybe you misunderstood :)

I was saying that if this improvement applies to applications,
it should be documented in the tuning guide.