Hello Peter On 18 February 2016 at 16:31, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote: > On 02/18/2016 03:25 AM, Michael Kerrisk (man-pages) wrote: >> Hello Peter, >> >> On 02/17/2016 05:37 PM, Peter Hurley wrote: [...] >> Okay -- so how's this text? >> >> In canonical mode: >> [...] >> * The maximum line length is 4096 chars (including the terminating >> newline character); lines longer than 4096 chars are truncated. >> After 4095 characters, input processing (e.g., ISIG and ECHO* >> processing) continues, but any input data after 4095 characters >> up to (but not including) any terminating newline is discarded. >> This ensures that the terminal can always receive more input >> until at least one line can be read. > > Yeah, this seems fine. Thanks for checking it. >>>>> However, the noncanonical mode input buffer size may change in the near >>>>> future. >>>> >>>> Can you say some more about that? >>> >>> At very high line rates and with ptys, the size of the read buffer becomes a >>> bottleneck, while at the same time, an already-committed page of memory is >>> going unused because of the page order allocation used by vm area allocator >>> (where N_TTY data is stored). >>> >>> The change is non-trivial however, since the maximum read size returned >>> by canonical mode cannot > 4096 bytes (because of the userspace assumptions >>> I referred to earlier). >>> >>>> And what changed in 3.13 with respect to noncanonical mode, by the way? >>> >>> In 3.12, the entire tty input path was parallelized. >>> >>> So while data is being read by userspace, input processing continues >>> concurrently, adding new data for userspace to read (the actual >>> construct used is a lockless circular buffer). In non-canonical modes, >>> the userspace copy is always performed twice -- first, from beginning of >>> unread data (head) to the end of buffer and then, from the beginning of >>> buffer to last unread data (tail). >>> >>> Since new data may be added to the circular buffer in parallel after the >>> first user copy has been performed, it's possible to retrieve up to 8192 >>> bytes (assuming the user copy buffer is that large). >> >> Thanks for the info. Do you think this warrants any changes in the man page? > > I don't think so. The goal of most changes in tty is to improve reliability > or performance w/o affecting userspace-observable behavior. Okay. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
