On Thu, Mar 1, 2012 at 3:29 PM, Dan Stromberg <drsalists(a)gmail.com> wrote: For a unique definition of "slow"... :) time. It's more based on obtaining and releasing resources. A slow system call is one that might take a long time, and any resource allocations requested during the system call processing can be easily undone (returned). TTY input, as an example. Things like a disk request are *supposed* to be quick, but even if they're not, they involve allocating buffers, queueing disk requests, setting up possible interrupts or DMA, and other messy internal details. Aborting a pending disk request would be a substantial amount of effort. Thus, a disk request cannot be interrupted and it's classified as "fast". (or "not slow") This presents a few difficulties when you consider something like NFS, which acts like a disk request until it gets down to the network layer, where things might well take a long time. But interrupting and aborting this is not easy, which is why some NFS requests, when hung, stay hung even when you bang on Control-C for a while. Really simple things like dup, which involves a simple copy, would probably not block unless there were extreme issues with memory allocation or something. -- Chris