ZFS LLC prefetch + ZFS

epower53

epower53

I'm in the midst of a server rebuild, and I'm puzzling over the LLC prefetch option in my Supermicro X11DPH-I BIOS. The default setting is disabled, but it sounds like it could be a useful feature to turn on. Any comments / experiences people can offer on whether this plays well with ZFS? More generally, how would enabling this option impact performance for a FreeBSD storage server running a few jails?
 
Ah, ok... for some reason I thought it was an extra-slow cache using main memory that might confuse the ARC. Any point in turning it on?
 
epower53 said:
Ah, ok... for some reason I thought it was an extra-slow cache using main memory that might confuse the ARC. Any point in turning it on?
Click to expand...

Some scientific computing workloads sound like something that could benefit. Maybe also video processing. Hard to predict.
 
It used to be on by default, but they went to off by default: https://cdrdv2-public.intel.com/780...th_Gen_Intel_Xeon_Scalable_Processor-R1_0.pdf

There are some (primarily single threaded and well behaved in temporal locality) programs that benefit, but I’m guessing the consensus (especially with the massive number of cores these days) was not enough, hence the change to default off. It creates extra memory bus traffic, so if you are memory-bound, the extra prefetch traffic tends to reduce performance. (Unless it gets very lucky and is mostly used before it gets evicted.)

ZFS and FreeBSD both use lots of cores (threads) to perform their IO work; I’m guessing no benefit would be seen, but I would love to see some benchmarks if you’re offering! ;)
 
Eric A. Borisch said:
I’m guessing no benefit would be seen, but I would love to see some benchmarks if you’re offering!
Click to expand...
Remind me in a few months... still getting my feet under me re-learning FreeBSD after many years. I'd like to get this machine up and running - once I a better feeling for how to administer a modern FreeBSD system I'd be happy to toggle the option in BIOS and do some head-to-head comparisons. My feeling is that I'll likely make a stupid mistake right now that would sour the results, vs a few months from now.
 
You must log in or register to reply here.
Back
Top