While it is fundamentally true that higher clock speed equals lower latency, I'm my practical experience we are more often interested in latency at the concurrency profile of the applications.
So in this regard I favor more cores when I have to choose, such that we can support more concurrent operations at a queue depth of 0.
Cheers
Wade
On Wed, Jan 20, 2016 at 7:58 AM Jan Schermer <jan@xxxxxxxxxxx> wrote:
I'm using Ceph with all SSDs, I doubt you have to worry about speed that
much with HDD (it will be abysmall either way).
With SSDs you need to start worrying about processor caches and memory
colocation in NUMA systems, linux scheduler is not really that smart right now.
Yes, the process will get its own core, but it might be a different core every
time it spins up, this increases latencies considerably if you start hammering
the OSDs on the same host.
But as always, YMMV ;-)
Jan
> On 20 Jan 2016, at 13:28, Oliver Dzombic <info@xxxxxxxxxxxxxxxxx> wrote:
>
> Hi Jan,
>
> actually the linux kernel does this automatically anyway ( sending new
> processes to "empty/low used" cores ).
>
> A single scrubbing/recovery or what ever process wont take more than
> 100% CPU ( one core ) because technically this processes are not able to
> run multi thread.
>
> Of course, if you configure your ceph to have ( up to ) 8 backfill
> processes, then 8 processes will start, which can utilize ( up to ) 8
> CPU cores.
>
> But still, the single process wont be able to use more than one cpu core.
>
> ---
>
> In a situation where you have 2x E5-2620v3 for example, you have 2x 6
> Cores x 2 HT Units = 24 Threads ( vCores ).
>
> So if you use inside such a system 24 OSD's every OSD will have (
> mathematically ) its "own" CPU Core automatically.
>
> Such a combination will perform better compared if you are using 1x E5
> CPU with a much higher frequency ( but still the same amout of cores ).
>
> This kind of CPU's are so fast, that the physical HDD ( no matter if
> SAS/SSD/ATA ) will not be able to overload the cpu ( no matter which cpu
> you use of this kind ).
>
> Its like if you are playing games. If the game is running smooth, it
> does not matter if its running on a 4 GHz machine on 40% utilization or
> on a 2 GHz machine with 80% utilization. Is running smooth, it can not
> do better :-)
>
> So if your data is coming as fast as the HDD can physical deliver it,
> its not important if the cpu runs with 2, 3, 4, 200 Ghz. Its already the
> max of what the HDD can deliver.
>
> So as long as the HDD's dont get faster, the CPU's does not need to be
> faster.
>
> The Ceph storage is usually just delivering data, not running a
> commercial webserver/what ever beside that.
>
> So if you are deciding what CPU you have to choose, you only have to
> think about how fast your HDD devices are. So that the CPU does not
> become the bottleneck.
>
> And the more cores you have, the lower is the chance, that different
> requests will block each other.
>
> ----
>
> So all in all, Core > Frequency, always. ( As long as you use fast/up to
> date CPUs ). If you are using old cpu's, of course you have to make sure
> that the performance of the cpu ( which does by the way not only depend
> on the frequency ) is sufficient that its not breaking the HDD data output.
>
>
>
> --
> Mit freundlichen Gruessen / Best regards
>
> Oliver Dzombic
> IP-Interactive
>
> mailto:info@xxxxxxxxxxxxxxxxx
>
> Anschrift:
>
> IP Interactive UG ( haftungsbeschraenkt )
> Zum Sonnenberg 1-3
> 63571 Gelnhausen
>
> HRB 93402 beim Amtsgericht Hanau
> Geschäftsführung: Oliver Dzombic
>
> Steuer Nr.: 35 236 3622 1
> UST ID: DE274086107
>
>
> Am 20.01.2016 um 13:10 schrieb Jan Schermer:
>> This is very true, but do you actually exclusively pin the cores to the OSD daemons so they don't interfere?
>> I don't think may people do that, it wouldn't work with more than a handful of OSDs.
>> The OSD might typicaly only need <100% of one core, but during startup or some reshuffling it's beneficial
>> to allow it to get more (>400%), and that will interfere with whatever else was pinned there...
>>
>> Jan
>>
>>> On 20 Jan 2016, at 13:07, Oliver Dzombic <info@xxxxxxxxxxxxxxxxx> wrote:
>>>
>>> Hi,
>>>
>>> Cores > Frequency
>>>
>>> If you think about recovery / scrubbing tasks its better when a cpu core
>>> can be assigned to do this.
>>>
>>> Compared to a situation where the same cpu core needs to recovery/scrub
>>> and still deliver the productive content at the same time.
>>>
>>> The more you can create a situation where an osd has its "own" cpu core,
>>> the better it is. Modern CPU's are anyway so fast, that even SSDs cant
>>> run the CPU's to their limit.
>>>
>>> --
>>> Mit freundlichen Gruessen / Best regards
>>>
>>> Oliver Dzombic
>>> IP-Interactive
>>>
>>> mailto:info@xxxxxxxxxxxxxxxxx
>>>
>>> Anschrift:
>>>
>>> IP Interactive UG ( haftungsbeschraenkt )
>>> Zum Sonnenberg 1-3
>>> 63571 Gelnhausen
>>>
>>> HRB 93402 beim Amtsgericht Hanau
>>> Geschäftsführung: Oliver Dzombic
>>>
>>> Steuer Nr.: 35 236 3622 1
>>> UST ID: DE274086107
>>>
>>>
>>> Am 20.01.2016 um 10:01 schrieb Götz Reinicke - IT Koordinator:
>>>> Hi folks,
>>>>
>>>> we plan to use more ssd OSDs in our first cluster layout instead of SAS
>>>> osds. (more IO is needed than space)
>>>>
>>>> short question: What would influence the performance more? more Cores or
>>>> more GHz/Core.
>>>>
>>>> Or is it as always: Depeds on the total of OSDs/nodes/repl-level/etc ... :)
>>>>
>>>> If needed, I can give some more detailed information on the layout.
>>>>
>>>> Thansk for feedback . Götz
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> ceph-users mailing list
>>>> ceph-users@xxxxxxxxxxxxxx
>>>> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
>>>>
>>> _______________________________________________
>>> ceph-users mailing list
>>> ceph-users@xxxxxxxxxxxxxx
>>> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
> _______________________________________________
> ceph-users mailing list
> ceph-users@xxxxxxxxxxxxxx
> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
_______________________________________________
ceph-users mailing list
ceph-users@xxxxxxxxxxxxxx
http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
_______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
