Re: heap_lock optimizations?

[Malcolm Crossley <malcolm.crossley@citrix.com>]

On 15/07/13 16:15, Konrad Rzeszutek Wilk wrote:
> Hey Tim,
>
> I was looking at making the 'Scrubbing Free RAM:' code faster on 1TB
> boxes with 128 CPUs. And naively I wrote code that setup a tasklet
> on each CPU and scrub a swatch of MFNs. Unfortunatly even on 8VCPU
> machines the end result was a slower boot time!
>
> The culprit looks to be the heap_lock that is taken and released
> on every MFN (for fun I added a bit of code to do batches - of
> 32 MFNs and to iterate over those 32 MFNs while holding the lock - that
> did make it a bit faster, but not by a much).
>
> What I am wondering is:
>   - Have you ever thought about optimizing this? If so, how?
>   - Another idea to potentially make this faster was to seperate this
>     scrubbing in two stages:
>      1) (under the heap_lock) - reserve/take a giant set of MFN pages
>         (perhaps also consult the NUMA affinity). This would be
>         usurping the whole heap[zone].
>      2) Give it out to the CPUS to scrub (this would be done without being
>         under a spinlock). The heap[zone] would be split equally amongst the
>         CPUs.
>      3) Goto 1 until done.
>   - Look for examples in the Linux kernel to see how it does it.
>
> Thanks!
Hi Konrad,

Did you see a patch I posted for this last year? 
http://lists.xen.org/archives/html/xen-devel/2012-05/msg00701.html

Unfortunately I made some minor errors and it didn't apply cleanly but 
I'll fix it up now and repost so you can test it.

Malcolm
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