Re: [Qemu-devel] [PATCH] block: add the support to drain throttled requests

[Stefan Hajnoczi]

On Fri, Mar 23, 2012 at 11:02 AM, Richard Davies <address@hidden> wrote:
> Stefan Hajnoczi wrote:
>> > Hi. We were producing the IDE assert()s and deadlocks with linux kernels.
>> > Although I believe the same symptoms exist on windows, I haven't actually
>> > tested it myself. Typically they would show up in the 16-bit bootloader
>> > code, even before the 32-bit OS has started.
>>
>> Okay, that makes sense.  Bootloaders and the BIOS may use the simplest
>> driver interface - which may be PIO in the case.  I asked because the
>> IDE DMA code path should work with I/O throttling and Windows is known
>> for sometimes falling back to the PIO code path when some heuristics
>> trigger.
>
> Whilst the bootloader was the easiest place for us to replicate this
> deadlock, we have also seen it with running Linux VMs.
>
> Older Linux kernels (e.g. CentOS 5) will fall back to PIO mode on IDE
> devices if they experience storage timeouts (e.g. due to heavy contention of
> underlying storage from other VMs).
>
> Hence, the IO limits deadlock can lead to running Linux VMs locking up as
> well as just Windows and Linux bootloaders.

Thanks for pointing out that Linux falls back too.

Stefan