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On DTrace envy

On DTrace envy

Posted Aug 8, 2007 2:06 UTC (Wed) by bgregg (subscriber, #46639)
In reply to: On DTrace envy by davem
Parent article: On DTrace envy

    Any predefined trace points are going to have their placement and semantics change on a week to week if not a day to day basis. It wouldn't help users at all.
I'm not sure I follow. That last sentence makes no sense to me at all.

The vast majority of people I've met (and also taught DTrace to), either don't have the time to browse kernel source and figure things out, or the assumed programming knowledge to do so. For example, if you had an NFS server latency issue and no stable probes, how many regular users would be able to pick their way through the NFS code, plus the TCP/IP stack code, plus the filesystem cache code, plus the disk driver code (or abstractions), and finally be able to point to the source of the latency? This would span around 300,000 lines of kernel code (in Solaris).

Stable probes allow users to trace complex systems without spending weeks untangling the code, or needing the programming background to understand the kernel. It also allows them to write stable scripts that will work for years to come. So, if the code is changing on a day to day basis, then there is an even greater need for these stable probes - if customers are expected to use the facility.

    whose pace of development is as glacial as Solaris's is
Much of the kernel does change frequently, such as the TCP/IP code. A couple of years ago I wrote some dynamic probe based scripts to monitor TCP traffic (tcpsnoop, etc), which have broken several times during those years due to kernel changes. It's a pain for customers trying to use them, and me trying to maintain them. The solution? I've written stable TCP/IP providers for Solaris (not yet putback), which will allow scripts similar to tcpsnoop to be both simple to write and robust.

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