Re: [RFC 0/4] Transparent on-demand struct page initializationembedded in the buddy allocator

[Ingo Molnar]

* Robin Holt <holt@xxxxxxx> wrote:

> [...]
> 
> With this patch, we did boot a 16TiB machine.  Without the patches, the 
> v3.10 kernel with the same configuration took 407 seconds for 
> free_all_bootmem.  With the patches and operating on 2MiB pages instead 
> of 1GiB, it took 26 seconds so performance was improved.  I have no feel 
> for how the 1GiB chunk size will perform.

That's pretty impressive.

It's still a 15x speedup instead of a 512x speedup, so I'd say there's 
something else being the current bottleneck, besides page init 
granularity.

Can you boot with just a few gigs of RAM and stuff the rest into hotplug 
memory, and then hot-add that memory? That would allow easy profiling of 
remaining overhead.

Side note:

Robert Richter and Boris Petkov are working on 'persistent events' support 
for perf, which will eventually allow boot time profiling - I'm not sure 
if the patches and the tooling support is ready enough yet for your 
purposes.

Robert, Boris, the following workflow would be pretty intuitive:

 - kernel developer sets boot flag: perf=boot,freq=1khz,size=16MB

 - we'd get a single (cycles?) event running once the perf subsystem is up
   and running, with a sampling frequency of 1 KHz, sending profiling
   trace events to a sufficiently sized profiling buffer of 16 MB per
   CPU.

 - once the system reaches SYSTEM_RUNNING, profiling is stopped either
   automatically - or the user stops it via a new tooling command.

 - the profiling buffer is extracted into a regular perf.data via a
   special 'perf record' call or some other, new perf tooling 
   solution/variant.

   [ Alternatively the kernel could attempt to construct a 'virtual'
     perf.data from the persistent buffer, available via /sys/debug or
     elsewhere in /sys - just like the kernel constructs a 'virtual' 
     /proc/kcore, etc. That file could be copied or used directly. ]

 - from that point on this workflow joins the regular profiling workflow: 
   perf report, perf script et al can be used to analyze the resulting
   boot profile.

Thanks,

	Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/