Figure 2-6. Oracle's instrumentation through an operating system lens. Oracle has submitted a multiblock IO read request to the operating system by issuing the operating system readv call. The call took 0.6 ms, which is wonderfully fast.

In Figure 2-6, the instrumented call is readv. But just before the Oracle kernel code makes the readv call, Oracle asks the operating system for the time by issuing the gettimeofday call. Then, immediately after the readv call, Oracle once again asks the operating system for the time by issuing another gettimeofday call. According to the Linux gettimeofday manual page, the second numeric output shown is a time in microseconds. Subtracting the initial time (801187) from the final time (801788), we find that this particular readv call took 601 (s (801788 - 801187), or 0.6 ms.

* According to the Linux manual page, readv is a synchronous call to the operating system that requests one or more blocks from an IO device. So, we know Oracle did not find these specific blocks in its buffer cache and was forced to ask the IO subsystem for the blocks. This is commonly called a physical IO or a block read.