Hi Stefan On Mon, May 21, 2012 at 11:20 PM, Stefan Hajnoczi <stefa...@gmail.com> wrote: > On Mon, May 21, 2012 at 12:00 PM, ronnie sahlberg > <ronniesahlb...@gmail.com> wrote: >> Yes, I use IDE since I boot from this LUN. >> >> >> I just managed to track it down to the IDE changes. >> >> It looks like basically this change triggered it : >> >> commit bef0fd5958120542f126f2dedbfce65d8839a94d >> Author: Stefan Hajnoczi <stefa...@linux.vnet.ibm.com> >> Date: Thu Mar 29 10:31:30 2012 +0100 >> ide: convert ide_sector_read() to asynchronous I/O >> >> >> Stefan, any ideas? Ill continue trying to understand the IDE code >> tomorrow and maybe I make some progress. > > It sounds like aio for iSCSI is broken. IDE submits bdrv_aio_readv() > requests which iSCSI handles. QEMU main loop should find that the > iSCSI socket file descriptor becomes readable when the SCSI response > arrives. We can then report I/O completion back to the guest. > > However, it appears that completions are not triggering. My guess is > that once the OS is fully booted you don't see the problem because > other events are pumping the main loop besides a 55ms timer. > Therefore this problem probably applies across the board but it's most > noticable when the BIOS/bootloader/early OS is executing.
I booted off a CDROM image and then had iscsi device as a disk. >From the cdrom booted guest I then did 'dd if=/dev/sda of=/dev/null bs=512 iflag=direct' but did not see these 55ms delays anymore. I did however see a ~4ms delay . > > Have you added debugging into block/iscsi.c or libiscsi to watch when > read handlers get called? Using tcpdump or strace might also be > useful to figure out when the reply becomes available and why we > aren't registering a read handler function with the QEMU main loop. I just did that, and I print a message and timeval to stdout for the functions in block/iscsi.c What seems to happen is that we call iscsi_aio_readv() which sets up a "write event" but this event is not triggered for another 55ms. Since the descriptor is writeable at this point, i would expected that the write_event_callback would trigger almost immediately. Below iscsi_aio_readv is printed when we enter this function. This function prepares a CDB and sets it up for queueing, but it does not actually write it to the socket. At the end of this function, we set up the events by calling iscsi_set_events() and we set it up for writeable. iscsi_process_write() is when we enter the fd-is-writeable callback. iscsi_aio_readv 1337676989.429822 iscsi_set_events write_event?Y 1337676989.429834 iscsi_process_write 1337676989.484424 iscsi_set_events write_event?N 1337676989.484507 iscsi_process_read 1337676989.484628 iscsi_aio_read10_cb 1337676989.484647 iscsi_set_events write_event?N 1337676989.484660 iscsi_readv_writev_bh_cb 1337676989.484665 iscsi_aio_readv 1337676989.484751 So what happens seems to be that once we have set up the i/o and queued it, and we set up the "writeable" event, but this event is not triggered immediately! It takes another 55ms before the event is triggered. To me this looks like a bug in the eventsystem. The socket is always writeable. shouldnt the callback be invoked almost immediately here ? regards ronnie sahlberg
