3 ms·

It is worth to point out that the CPUs have much more pronounced hiccups like SMIs (system management interrupts) on Intel ones. These can be turned off at the

It is worth to point out that the CPUs have much more pronounced hiccups like SMIs (system management interrupts) on Intel ones. These can be turned off at the cost of making the server able to actually overheat (SMIs are for example used to check if the CPU is on fire).

bonzini 8y ago
Good servers do not need periodic SMIs, stuff like temperature sensors are handled by the BMC. Of course it depends on the benches and it can be a pain to find them, but people using the Linux realtime patch for example really need servers that do but abuse SMIs. Periodic SMIs in theory should only be used for stupid stuff like emulating a PS/2 keyboard and mouse from a USB one, which are safe to disable. SMIs are used to handle machine checks, access to persistent storage (including UEFI variables), and various RAS (reliability/availability/serviceability) features.
- lmilcin 8y ago
  Well, I don't know for a fact everything those interrupts do. What I know is that I had to turn those off (along with bunch of other things) to meet strict realtime guarantees for my proof of concept algorihmic trading framework I did for a brokerage house. This was few years back on Haswell on best hardware you could buy including top bin Xeons dedicated for algotrading that were clocking 5GHz by default (frequency locked, sleep states turned off, etc). On the other hand I never noticed anything funky with regards to memory latency that would cause me to investigate. This is probably because I would already treat memory like a remote database and try to do as much as possible within L2 and L3. The budget for entire transaction (from the moment bytes reached NIC to the moment bytes left NIC as measured by external switch) was 5us so 0.1us was below noise threshold.
  - bonzini 8y ago
    Yes, you have to turn the periodic SMI off but it's not a problem with respect to overheating or detecting memory errors.

bonzini 8y ago

Good servers do not need periodic SMIs, stuff like temperature sensors are handled by the BMC. Of course it depends on the benches and it can be a pain to find them, but people using the Linux realtime patch for example really need servers that do but abuse SMIs. Periodic SMIs in theory should only be used for stupid stuff like emulating a PS/2 keyboard and mouse from a USB one, which are safe to disable. SMIs are used to handle machine checks, access to persistent storage (including UEFI variables), and various RAS (reliability/availability/serviceability) features.

lmilcin 8y ago
Well, I don't know for a fact everything those interrupts do. What I know is that I had to turn those off (along with bunch of other things) to meet strict realtime guarantees for my proof of concept algorihmic trading framework I did for a brokerage house. This was few years back on Haswell on best hardware you could buy including top bin Xeons dedicated for algotrading that were clocking 5GHz by default (frequency locked, sleep states turned off, etc). On the other hand I never noticed anything funky with regards to memory latency that would cause me to investigate. This is probably because I would already treat memory like a remote database and try to do as much as possible within L2 and L3. The budget for entire transaction (from the moment bytes reached NIC to the moment bytes left NIC as measured by external switch) was 5us so 0.1us was below noise threshold.
- bonzini 8y ago
  Yes, you have to turn the periodic SMI off but it's not a problem with respect to overheating or detecting memory errors.