On Wed, Mar 07, 2012 at 09:39:50PM -0500, Zach Brown wrote: > > >Can you devise a non-secret testcase that demonstrates this? > > Hmm. I bet you could get fio to do it. Giant file, random mmap() > writes, spin until the CPU overwhelms writeback? Kick off a bunch of fio processes, each in separate I/O cgroups set up so that each of the processes get a "fair" amount of the I/O bandwidth. (This is quite common in cloud deployments where you are packing a huge number of tasks onto a single box; whether the tasks are inside virtual machines or containers don't really matter for the purpose of this exercise. We basically need to simulate a system where the disks are busy.) Then in one of those cgroups, create a process which is constantly appending to a file using buffered I/O; this could be a log file, or an application-level journal file; and measure the latency of that write system call. Every so often, writeback will push the dirty pages corresponding to the log/journal file to disk. When that happens, and page stablization is enabled, the latency of that write system call will spike. And any time you have a distributed system where you are depending on a large number of RPC/SOAP/Service Oriented Architecture Enterpise Service Bus calls (I don't really care which buzzword you use, but IBM and Oracle really like the last one :-), long-tail latencies are what kill your responsiveness and predictability. Especially when a thread goes away for a second or more... - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
