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Re: per-entity statistics


Jose R. Santos wrote:
This is one of the scenarios we are designing our trace tool for. While it does not meet you requirement to just probe a selected number of devices, something like that would be easy to hack into the current implementation since we already take SCSI host number, channel, lun and id. We also have hooks for the IO schedulers and system call and have plans to put hooks into SCSI drivers. For some of the workloads that we are want use this tool, we need to be able to measure latencies from the moment the IO was submitted by the application.

The reason why I want probes to be able to select devices to be scrutinized is to make sure that gathering statistics impacts performance as little as possible.

Other ways to reduce probe overhead for simple latency measurements
include reducing the frequencing of stuff being gathered and reported
to userspace, and reducing the amount of data sampled for each event,
I guess. Systemtap's statistics seem to fit these requirements
quite well, as long as the instant aggregation doesn't become more
expensive than some arithmetic operations or comparisons.

It might be feasible to add some device selection code to the trace
tool. However, I feel the trace tool would still do more than needed
for simple latency measurements, even if it supported device selection.

The trace tool might be advantageous when placing several time related
probes in order to measure an entire stack of delay components.
I am not sure what the universal answer regarding the performance
analysis question is, and whether there can be one at all.
I think, I will just try to hack up some prototype doing device
selection and using systemtap-style statistics.

Thanks for your thoughts, anyway. I am curious to see how the trace
tool performs and what kind of hickups it will detect.

Martin


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