On 02/08/2011 05:15 AM, Aurelien Jarno wrote:
Anthony Liguori a écrit :
On 02/08/2011 04:06 AM, Aurelien Jarno wrote:
Yes, it's slow. But is it a problem? You assume that people use QEMU
only for emulating SMP platforms. This is a wrong assumption. Beside the
x86 target, only sparc really supports SMP emulation.
It's *not* just about performance.
TCG requires a signal to break out of a tight chained TB loop. If you
have a guest in a tight loop waiting for something external (like
polling on a in-memory flag), the device emulation will not get to run
until a signal is fired.
Unless you set SIGIO on every file descriptor that selects polls on (and
you can't because there are a number that just don't support SIGIO),
then you have a race condition.
In practice you will get a signal when the next timer event expire. I
agree it's suboptimal, but it works, and has been like that for here.
During early boot up before the periodic timer is enabled can cause
quite a noticable issue here.
I think it's cris specifically that does polling I/O in the early
startup before any periodic timer is enabled.
Having that fixed through an I/O thread is actually quite nice, however
it should not be done ignoring all the *current* drawbacks of the
iothread mode. We know them (at least for some of them), so let's try to
solve them.
Yes, agree 100%.
And now, I don't buy the argument "it's been there for years", it was
*disabled* by default.
Yeah, I think we need to enable it by default and commit to fixing all
of the outstanding issues.
I think we've fixed all that we're aware of but we probably won't find
the rest unless we enable it universally.
Regards,
Anthony Liguori
