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[Qemu-devel] Re: [PATCH 2/3] Fix time drift problem under high load when
From: |
Jan Kiszka |
Subject: |
[Qemu-devel] Re: [PATCH 2/3] Fix time drift problem under high load when PIT is in use. |
Date: |
Sun, 29 Jun 2008 16:40:43 +0200 |
User-agent: |
Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv:1.8.1.12) Gecko/20080226 SUSE/2.0.0.12-1.1 Thunderbird/2.0.0.12 Mnenhy/0.7.5.666 |
Gleb Natapov wrote:
> Count the number of interrupts that was lost due to interrupt coalescing
> and re-inject them back when possible. This fixes time drift problem when
> pit is used as a time source.
>
> Signed-off-by: Gleb Natapov <address@hidden>
> ---
>
> hw/i8254.c | 20 +++++++++++++++++++-
> 1 files changed, 19 insertions(+), 1 deletions(-)
>
> diff --git a/hw/i8254.c b/hw/i8254.c
> index 4813b03..c4f0f46 100644
> --- a/hw/i8254.c
> +++ b/hw/i8254.c
> @@ -61,6 +61,8 @@ static PITState pit_state;
>
> static void pit_irq_timer_update(PITChannelState *s, int64_t current_time);
>
> +static uint32_t pit_irq_coalesced;
> +
> static int pit_get_count(PITChannelState *s)
> {
> uint64_t d;
> @@ -369,12 +371,28 @@ static void pit_irq_timer_update(PITChannelState *s,
> int64_t current_time)
> return;
> expire_time = pit_get_next_transition_time(s, current_time);
> irq_level = pit_get_out1(s, current_time);
> - qemu_set_irq(s->irq, irq_level);
> + if(irq_level) {
> + if(!qemu_irq_raise(s->irq))
> + pit_irq_coalesced++;
> + } else {
> + qemu_irq_lower(s->irq);
> + if(pit_irq_coalesced > 0) {
> + if(qemu_irq_raise(s->irq))
> + pit_irq_coalesced--;
> + qemu_irq_lower(s->irq);
> + }
> + }
That's graspable for my poor brain: reinject one coalesced IRQ right
after the falling edge of an in-time delivery...
> +
> #ifdef DEBUG_PIT
> printf("irq_level=%d next_delay=%f\n",
> irq_level,
> (double)(expire_time - current_time) / ticks_per_sec);
> #endif
> + if(pit_irq_coalesced && expire_time != -1) {
> + uint32_t div = ((pit_irq_coalesced >> 10) & 0x7f) + 2;
> + expire_time -= ((expire_time - current_time) / div);
> + }
> +
... but could you comment on this algorithm? I think I got what it does:
splitting up the next regular period in short intervals. But there are a
bit too many magic numbers involved. Where do they come from? What
happens if pit_irq_coalesced becomes large (or: how large can it become
without risking an IRQ storm on the guest)? A few comments would help, I
guess.
Thanks,
Jan
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- Re: [Qemu-devel] [PATCH 1/3] Change qemu_set_irq() to return status information., (continued)
- [Qemu-devel] Re: [PATCH 1/3] Change qemu_set_irq() to return status information., Jan Kiszka, 2008/06/29
- Re: [Qemu-devel] Re: [PATCH 1/3] Change qemu_set_irq() to return status information., Paul Brook, 2008/06/29
- Re: [Qemu-devel] Re: [PATCH 1/3] Change qemu_set_irq() to return status information., Dor Laor, 2008/06/29
- Re: [Qemu-devel] Re: [PATCH 1/3] Change qemu_set_irq() to return status information., Jan Kiszka, 2008/06/29
- Re: [Qemu-devel] Re: [PATCH 1/3] Change qemu_set_irq() to return status information., Paul Brook, 2008/06/29
- [Qemu-devel] Re: [PATCH 1/3] Change qemu_set_irq() to return status information., Gleb Natapov, 2008/06/30
[Qemu-devel] [PATCH 2/3] Fix time drift problem under high load when PIT is in use., Gleb Natapov, 2008/06/29
[Qemu-devel] [PATCH 3/3] Fix time drift problem under high load when RTC is in use., Gleb Natapov, 2008/06/29
[Qemu-devel] Re: [PATCH 0/3] Fix guest time drift under heavy load., Jan Kiszka, 2008/06/29