Re: RFC: auto-enabling parallel-checkout on NFS

[Jeff Hostetler <git@jeffhostetler.com>]

On 11/18/20 11:01 PM, Matheus Tavares wrote:
> Hi, Jeff
> 
> On Mon, Nov 16, 2020 at 12:19 PM Jeff Hostetler <git@jeffhostetler.com> wrote:
>>
>> I can't really speak to NFS performance, but I have to wonder if there's
>> not something else affecting the results -- 4 and/or 8 core results are
>> better than 16+ results in some columns.  And we get diminishing returns
>> after ~16.
> 
> Yeah, that's a good point. I'm not sure yet what's causing the
> diminishing returns, but Geert and I are investigating. Maybe we are
> hitting some limit for parallelism in this scenario.

I seem to recall back when I was working on this problem that
the unzip of each blob was a major pain point.  Combine this
long delta-chains and each worker would need multiple rounds of
read/memmap, unzip, and de-delta before it had the complete blob
and could then smudge and write.

This makes me wonder if repacking the repo with shorter delta-chains
affects the checkout times.  And improves the perf when there are
more workers.  I'm not saying that this is a solution, but rather
an experiment to see if it changes anything and maybe adjust our
focus.

> 
>> I'm wondering if during these test runs, you were IO vs CPU bound and if
>> VM was a problem.
> 
> I would say we are more IO bound during these tests. While a sequential
> linux-v5.8 checkout usually uses 100% of one core in my laptop's SSD,
> in this setup, it only used 5% to 10%. And even with 64 workers (on a
> single core), CPU usage stays around 60% most of the time.
> 
> About memory, the peak PSS was around 1.75GB, with 64 workers, and the
> machine has 10GB of RAM. But are there other numbers that I should keep
> an eye on while running the test?
> 
>> I'm wondering if setting thread affinity would help here.
> 
> Hmm, I only had one core online during the benchmark, so I think thread
> affinity wouldn't impact the runtime.

I wasn't really thinking about the 64 workers on 1 core case.  I was
more thinking about the 64 workers on 64 cores case and wondering
if workers were being randomly bounced from core to core and we were
thrashing.

> 
> Thanks,
> Matheus
>