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

[Robert Richter <rric@kernel.org>]

On 12.07.13 10:27:56, Ingo Molnar wrote:
> 
> * Robin Holt <holt@sgi.com> 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.

The latest patch set is still this:

 git://git.kernel.org/pub/scm/linux/kernel/git/rric/oprofile.git persistent-v2

It requires the perf subsystem to be initialized first which might be
too late, see perf_event_init() in start_kernel(). The patch set is
currently also limited to tracepoints only.

If this is sufficient for you, you might register persistent events
with the function perf_add_persistent_event_by_id(), see
mcheck_init_tp() how to do this. Later you can fetch all samples with:

 # perf record -e persistent/<tracepoint>/ sleep 1

> 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.

I am not sure about the event you want to setup here, if it is a
tracepoint the sample_period should be always 1. The buffer size
parameter looks interesting, for now it is 512kB per cpu per default
(as perf tools setup the buffer).

> 
>  - 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.

See the perf-record command above...

> 
>    [ 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.

Ingo, thanks for outlining this workflow. We will look how this could
fit into the new version of persistent events we currently working on.

Thanks,

-Robert

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