On Sun, Jun 23, 2013 at 11:28:40AM +0200, Ingo Molnar wrote:
>
> That's 4.5 GB/sec initialization speed - that feels a bit slow and the
> boot time effect should be felt on smaller 'a couple of gigabytes' desktop
> boxes as well. Do we know exactly where the 2 hours of boot time on a 32
> TB system is spent?
>
There are other several spots that could be improved on a large system but
memory initialization is by far the biggest.
> While you cannot profile the boot process (yet), you could try your
> delayed patch and run a "perf record -g" call-graph profiling of the
> late-time initialization routines. What does 'perf report' show?
>
I have some data from earlier runs.
memmap_init_zone was the function that was the biggest hitter by far.
Parts of it could certianly are low hanging fruit, set_pageblock_migratetype
for example.
However it seems for a larger system SetPageReserved will be the largest
consumer of cycles. On a 1TB system I just booted it was around 50% of time
spent in memmap_init_zone.
perf seems to struggle with 512 cpus, but I did get some data.
It seems to indicate similar data to what I found in earlier experiments.
Lots of time in memmap_init_zone,
Some are waiting on locks, this guy seems to be representative of that.
- 0.14% kworker/160:1 [kernel.kallsyms] [k] mspin_lock ▒
+ mspin_lock ▒
+ __mutex_lock_slowpath ▒
- mutex_lock ▒
- 99.69% online_pages
> Delayed initialization makes sense I guess because 32 TB is a lot of
> memory - I'm just wondering whether there's some low hanging fruits left
> in the mem init code, that code is certainly not optimized for
> performance.
>
> Plus with a struct page size of around 64 bytes (?) 32 TB of RAM has 512
> GB of struct page arrays alone. Initializing those will take quite some
> time as well - and I suspect they are allocated via zeroing them first. If
> that memset() exists then getting rid of it might be a good move as well.
>
> Yet another thing to consider would be to implement an initialization
> speedup of 3 orders of magnitude: initialize on the large page (2MB)
> grandularity and on-demand delay the initialization of the 4K granular
> struct pages [but still allocating them] - which I suspect are a good
> chunk of the overhead? That way we could initialize in 2MB steps and speed
> up the 2 hours bootup of 32 TB of RAM to 14 seconds...
>
> [ The cost would be one more branch in the buddy allocator, to detect
> not-yet-initialized 2 MB chunks as we encounter them. Acceptable I
> think. ]
>
> Thanks,
>
> Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-doc" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
