Re: [RFC PATCH 0/14] Parallel memory initialisation

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On Thu, Apr 16, 2015 at 3:51 PM, Daniel J Blueman <daniel@xxxxxxxxxxxxx> wrote:
On Monday, April 13, 2015 at 6:20:05 PM UTC+8, Mel Gorman wrote:
> Memory initialisation had been identified as one of the reasons why large
> machines take a long time to boot. Patches were posted a long time ago
> that attempted to move deferred initialisation into the page allocator
> paths. This was rejected on the grounds it should not be necessary to hurt
> the fast paths to parallelise initialisation. This series reuses much of
> the work from that time but defers the initialisation of memory to kswapd
> so that one thread per node initialises memory local to that node. The
> issue is that on the machines I tested with, memory initialisation was not
> a major contributor to boot times. I'm posting the RFC to both review the
> series and see if it actually helps users of very large machines.
> After applying the series and setting the appropriate Kconfig variable I
> see this in the boot log on a 64G machine
> [    7.383764] kswapd 0 initialised deferred memory in 188ms
> [    7.404253] kswapd 1 initialised deferred memory in 208ms
> [    7.411044] kswapd 3 initialised deferred memory in 216ms
> [    7.411551] kswapd 2 initialised deferred memory in 216ms
> On a 1TB machine, I see
> [   11.913324] kswapd 0 initialised deferred memory in 1168ms
> [   12.220011] kswapd 2 initialised deferred memory in 1476ms
> [   12.245369] kswapd 3 initialised deferred memory in 1500ms
> [   12.271680] kswapd 1 initialised deferred memory in 1528ms
> Once booted the machine appears to work as normal. Boot times were measured
> from the time shutdown was called until ssh was available again.  In the
> 64G case, the boot time savings are negligible. On the 1TB machine, the
> savings were 10 seconds (about 8% improvement on kernel times but 1-2%
> overall as POST takes so long).
> It would be nice if the people that have access to really large machines
> would test this series and report back if the complexity is justified.

Nice work!

On an older Numascale system with 1TB memory and 256 cores/32 NUMA nodes, platform init takes 52s (cold boot), firmware takes 84s (includes one warm reboot), stock linux 4.0 then takes 732s to boot [1] (due to the 700ns roundtrip, RMW cache-coherent cycles due to the temporal writes for pagetable init and per-core store queue limits), so there is huge potential.

Same 1TB setup (256 cores, 32 NUMA nodes):
unpatched 4.0: 789s [1]
2GB per node up-front: 426s [2]
4GB node 0 up-front, 0GB later nodes: 461s [3]
4GB node 0 up-front, 0.5GB later nodes: 404s [4]

Compelling results at only 1TB! In the last case, we see PMD setup take 42% (168s) of the time, along with topology_init taking 39% (157s). I should be able to get data on a 7TB system this week.

[1] https://resources.numascale.com/telemetry/defermem/h8qgl-defer-stock.txt
[2] https://resources.numascale.com/telemetry/defermem/h8qgl-defer-2g.txt
[3] https://resources.numascale.com/telemetry/defermem/h8qgl-defer-4+0.txt
[4] https://resources.numascale.com/telemetry/defermem/h8qgl-defer-4+half.txt

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