Question

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Pipe buffer size is 4k or 64k?

1. In linux, which header file specifies the size available for writes on a pipe ?

2. I capture latency of my main application per configurable cycle and write that data to a pipe. A separate reporting process reads off that pipe. Typically, the main application exchanges about 10,000 messages per second. So, given a cycle of one second, the main application collects 10k latency data points for each message exchange and then writes them off to a pipe on a second’s boundary. I have following questions in this scenario

   • Is there way to specify the size of pipe while creation,so i can ensure there is adequate write space in the pipe?
   • Are writes to pipe expensive? How is pipe implemented? Do writes to pipe go against some mmap file or in-memory buffer?

Answer 1

• Is there way to specify the size of a pipe at creation? Maybe. Since Linux 2.6.35 you can use fcntl(2) with the F_SETPIPE_SZ operation to set the pipe buffer up to /proc/sys/fs/pipe-max-size. On earlier versions, no, but I suppose you could use the socket mechanism instead. It would be slower for most purposes but you can specify the amount of buffering up to wmem_max, see socket(7), and you have certain other controls over kernel memory allocation.
• Are writes to pipe expensive? No. But write(2) is a kernel call, so pipe I/O should be buffered if possible.
• How are pipes implemented? With kernel code that transfers data in and out of the system buffer cache.
• Do writes to pipe go against some mmap file or in-memory buffer? It’s an in-memory buffer