Messages in this thread | | | Date | Mon, 12 Dec 2011 21:14:25 +0200 | Subject | Re: [patch 0/3] kvm tool: Serial emulation overhaul | From | Pekka Enberg <> |
| |
On Mon, Dec 12, 2011 at 7:20 PM, Ingo Molnar <mingo@elte.hu> wrote: >>> I'm seeing 1.5 usecs per character for this little benchmark: >> >> Interesting - what do you see with the top -b measurement i >> posted - can you see similar slowdowns?
On Mon, Dec 12, 2011 at 8:40 PM, Pekka Enberg <penberg@kernel.org> wrote: > ssh-4.2# time top -d 0.01 -n 10 -b > > [snip] > > real 0m2.935 > user 0m0.006s > sys 0m0.049s > > That's 9.7 usecs per character which is roughly 6x slowdown. > > It seems to be related to interrupt handling because if I bump up > TIMER_INTEVAL_NS to 10 msec:
[snip]
Looking at drivers/tty/serial/8250.c, we have:
static void serial8250_console_putchar(struct uart_port *port, int ch) { struct uart_8250_port *up = container_of(port, struct uart_8250_port, port);
wait_for_xmitr(up, UART_LSR_THRE); serial_out(up, UART_TX, ch); }
in tools/kvm/hw/serial.c::serial8250_out() we do:
case UART_TX: if (!(dev->lcr & UART_LCR_DLAB)) { char *addr = data;
if (!(dev->mcr & UART_MCR_LOOP)) term_putc(CONSOLE_8250, addr, size, dev->id); /* else FIXME: Inject data into rcv path for LOOP */
if (++dev->txcnt == 16) dev->lsr &= ~(UART_LSR_TEMT | UART_LSR_THRE); break;
which clears UART_LSR_THRE after 16 characters. UART_LSR_THRE is only enabled in serial8250__update_consoles() and serial8250_update_irq() which are completely bound by TIMER_INTERVAL_NS.
Pekka -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |