From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=33710 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OHiRS-0005ny-LA for qemu-devel@nongnu.org; Thu, 27 May 2010 15:08:32 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OHiRP-0006P2-Rf for qemu-devel@nongnu.org; Thu, 27 May 2010 15:08:29 -0400 Received: from fmmailgate03.web.de ([217.72.192.234]:40949) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OHiRP-0006On-D0 for qemu-devel@nongnu.org; Thu, 27 May 2010 15:08:27 -0400 Message-ID: <4BFEC322.3030207@web.de> Date: Thu, 27 May 2010 21:08:18 +0200 From: Jan Kiszka MIME-Version: 1.0 Subject: Re: [Qemu-devel] Re: [RFT][PATCH 07/15] qemu_irq: Add IRQ handlers with delivery feedback References: <4BFD8010.3010001@web.de> <201005270026.34119.paul@codesourcery.com> <4BFEBA66.6030804@web.de> In-Reply-To: Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enigFB2BED90AEB2D4DB7F30BA03" Sender: jan.kiszka@web.de List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Blue Swirl Cc: Juan Quintela , Paul Brook , qemu-devel@nongnu.org This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enigFB2BED90AEB2D4DB7F30BA03 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Blue Swirl wrote: > On Thu, May 27, 2010 at 6:31 PM, Jan Kiszka wrote: >> Blue Swirl wrote: >>> On Wed, May 26, 2010 at 11:26 PM, Paul Brook = wrote: >>>>> At the other extreme, would it be possible to make the educated gue= sts >>>>> aware of the virtualization also in clock aspect: virtio-clock? >>>> The guest doesn't even need to be aware of virtualization. It just n= eeds to be >>>> able to accommodate the lack of guaranteed realtime behavior. >>>> >>>> The fundamental problem here is that some guest operating systems as= sume that >>>> the hardware provides certain realtime guarantees with respect to ex= ecution of >>>> interrupt handlers. In particular they assume that the CPU will alw= ays be >>>> able to complete execution of the timer IRQ handler before the perio= dic timer >>>> triggers again. In most virtualized environments you have absolutel= y no >>>> guarantee of realtime response. >>>> >>>> With Linux guests this was solved a long time ago by the introductio= n of >>>> tickless kernels. These separate the timekeeping from wakeup events= , so it >>>> doesn't matter if several wakeup triggers end up getting merged (eit= her at the >>>> hardware level or via top/bottom half guest IRQ handlers). >>>> >>>> >>>> It's worth mentioning that this problem also occurs on real hardware= , >>>> typically due to lame hardware/drivers which end up masking interrup= ts or >>>> otherwise stall the CPU for for long periods of time. >>>> >>>> >>>> The PIT hack attempts to workaround broken guests by adding artifici= al latency >>>> to the timer event, ensuring that the guest "sees" them all. Unfort= unately >>>> guests vary on when it is safe for them to see the next timer event,= and >>>> trying to observe this behavior involves potentially harmful heurist= ics and >>>> collusion between unrelated devices (e.g. interrupt controller and t= imer). >>>> >>>> In some cases we don't even do that, and just reschedule the event s= ome >>>> arbitrarily small amount of time later. This assumes the guest to do= useful >>>> work in that time. In a single threaded environment this is probably= true - >>>> qemu got enough CPU to inject the first interrupt, so will probably = manage to >>>> execute some guest code before the end of its timeslice. In an envir= onment >>>> where interrupt processing/delivery and execution of the guest code = happen in >>>> different threads this becomes increasingly likely to fail. >>> So any voodoo around timer events is doomed to fail in some cases. >>> What's the amount of hacks what we want then? Is there any generic >> The aim of this patch is to reduce the amount of existing and upcoming= >> hacks. It may still require some refinements, but I think we haven't >> found any smarter approach yet that fits existing use cases. >=20 > I don't feel we have tried other possibilities hard enough. Well, seeing prototypes wouldn't be bad, also to run real load againt them. But at least I'm currently clueless what to implement. >=20 >>> solution, like slowing down the guest system to the point where we ca= n >>> guarantee the interrupt rate vs. CPU execution speed? >> That's generally a non-option in virtualized production environments. >> Specifically if the guest system lost interrupts due to host >> overcommitment, you do not want it slow down even further. >=20 > I meant that the guest time could be scaled down, for example 2s in > wall clock time would be presented to the guest as 1s. But that is precisely what already happens when the guest loses timer interrupts. There is no other time source for this kind of guests - often except for some external events generated by systems which you don't want to fall behind arbitrarily. > Then the amount > of CPU cycles between timer interrupts would increase and hopefully > the guest can keep up. If the guest sleeps, time base could be > accelerated to catch up with wall clock and then set back to 1:1 rate. Can't follow you ATM, sorry. What should be slowed down then? And how precisely? Jan >=20 > Slowing down could be triggered by measuring the guest load (for > example, by checking for presence of halt instructions), if it's close > to 1, time would be slowed down. If the guest starts to issue halt > instructions because it's more idle, we can increase speed. >=20 > If this approach worked, even APIC could be made ignorant about > coalescing voodoo so it should be a major cleanup. --------------enigFB2BED90AEB2D4DB7F30BA03 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.9 (GNU/Linux) Comment: Using GnuPG with SUSE - http://enigmail.mozdev.org iEYEARECAAYFAkv+wygACgkQitSsb3rl5xTIcQCgjr426TvdaRf3b1UTy536AGOy LCIAoIJCN0Jp5g2DzPuKpws7ql+nfu31 =NgPS -----END PGP SIGNATURE----- --------------enigFB2BED90AEB2D4DB7F30BA03--