Re: [RFC PATCH 0/5] Memory access profiler(IBS) driven NUMA balancing

[Huang, Ying]

Bharata B Rao <bharata@xxxxxxx> writes:

> On 27-Feb-23 1:24 PM, Huang, Ying wrote:
>> Thank you very much for detailed data.  Can you provide some analysis
>> for your data?
>
> The overhead numbers I shared earlier weren't correct as I
> realized that while obtaining those numbers from function_graph
> tracing, the trace buffer was silently getting overrun. I had to
> reduce the number of memory access iterations to ensure that I get
> the full trace buffer. I will be summarizing the findings
> based on this new numbers below.
>
> Just to recap - The microbenchmark is run on an AMD Genoa
> two node system. The benchmark has two set of threads,
> (one affined to each node) accessing two different chunks
> of memory (chunk size 8G) which are initially allocated
> on first node. The benchmark touches each page in the
> chunk iteratively for a fixed number of iterations (384
> in this case given below). The benchmark score is the
> amount of time it takes to complete the specified number
> of accesses.
>
> Here is the data for the benchmark run:
>
> Time taken or overhead (us) for fault, task_work and sched_switch
> handling
>
> 				Default		IBS
> Fault handling			2875354862	2602455		
> Task work handling		139023		24008121
> Sched switch handling				37712
> Total overhead			2875493885	26648288	
>
> Default
> -------
> 			Total		Min	Max		Avg
> do_numa_page		2875354862	0.08	392.13		22.11
> task_numa_work		139023		0.14	5365.77		532.66
> Total			2875493885
>
> IBS
> ---
> 			Total		Min	Max		Avg
> ibs_overflow_handler	2602455		0.14	103.91		1.29
> task_ibs_access_work	24008121	0.17	485.09		37.65
> hw_access_sched_in	37712		0.15	287.55		1.35
> Total			26648288
>
>
> 				Default		IBS
> Benchmark score(us)		160171762.0	40323293.0
> numa_pages_migrated		2097220		511791
> Overhead per page		1371		52
> Pages migrated per sec		13094		12692
> numa_hint_faults_local		2820311		140856
> numa_hint_faults		38589520	652647

For default, numa_hint_faults >> numa_pages_migrated.  It's hard to be
understood.  I guess that there aren't many shared pages in the
benchmark?  And I guess that the free pages in the target node is enough
too?

> hint_faults_local/hint_faults	7%		22%
>
> Here is the summary:
>
> - In case of IBS, the benchmark completes 75% faster compared to
>   the default case. The gain varies based on how many iterations of
>   memory accesses we run as part of the benchmark. For 2048 iterations
>   of accesses, I have seen a gain of around 50%.
> - The overhead of NUMA balancing (as measured by the time taken in
>   the fault handling, task_work time handling and sched_switch time
>   handling) in the default case is seen to be pretty high compared to
>   the IBS case.
> - The number of hint-faults in the default case is significantly
>   higher than the IBS case.
> - The local hint-faults percentage is much better in the IBS
>   case compared to the default case.
> - As shown in the graphs (in other threads of this mail thread), in
>   the default case, the page migrations start a bit slowly while IBS
>   case shows steady migrations right from the start.
> - I have also shown (via graphs in other threads of this mail thread)
>   that in IBS case the benchmark is able to steadily increase
>   the access iterations over time, while in the default case, the
>   benchmark doesn't do forward progress for a long time after
>   an initial increase.

Hard to understand this too.  Pages are migrated to local, but
performance doesn't improve.

> - Early migrations due to relevant access sampling from IBS,
>   is most probably the significant reason for the uplift that IBS
>   case gets.

In original kernel, the NUMA page table scanning will delay for a
while.  Please check the below comments in task_tick_numa().

	/*
	 * Using runtime rather than walltime has the dual advantage that
	 * we (mostly) drive the selection from busy threads and that the
	 * task needs to have done some actual work before we bother with
	 * NUMA placement.
	 */

I think this is generally reasonable, while it's not best for this
micro-benchmark.

Best Regards,
Huang, Ying

> - It is consistently seen that the benchmark in the IBS case manages
>   to complete the specified number of accesses even before the entire
>   chunk of memory gets migrated. The early migrations are offsetting
>   the cost of remote accesses too.
> - In the IBS case, we re-program the IBS counters for the incoming
>   task in the sched_switch path. It is seen that this overhead isn't
>   that significant to slow down the benchmark.
> - One of the differences between the default case and the IBS case
>   is about when the faults-since-last-scan is updated/folded into the
>   historical faults stats and subsequent scan period update. Since we
>   don't have the notion of scanning in IBS, I have a threshold (number
>   of access faults) to determine when to update the historical faults
>   and the IBS sample period. I need to check if quicker migrations
>   could result from this change.
> - Finally, all this is for the above mentioned microbenchmark. The
>   gains on other benchmarks is yet to be evaluated.
>
> Regards,
> Bharata.