Understanding clock drift in virtual machines w/ CFS, NOHZ
Good day. I have come across the occasional issue of clock drift when running (earlier) kernels inside a virtual machine. The reason for drift is simple enough - host may be under heavy load, guest may not get enough resources to run all the ticks (HZ) it is supposed to. At least to put it simply. Now, I have been wondering how the introduction of CFS and CONFIG_NOHZ (dynticks) change this, or if they do at all. If I have understood correctly, only the host's scheduler is really involved so it enters the picture when host is running a recent kernel. In a pure hypervisor-only virtualization, that point should be moot. And then we have dynticks. Try as I might, I haven't been able to wrap my head around the combination. What happens to the kernel and timekeeping when guest has NOHZ enabled? My google-fu is not good enough to find the relevant documentation if such even exists on this subject. To top it off, I'm not certain whether my question is trivial, non-issue or just plain weird. Any pointers on where I should look for more information will be appreciated. -- Mika Boström +358-40-525-7347 -=- The flogging will continue [EMAIL PROTECTED]www.iki.fi/bostik -=- until morale improves GPG: 0x039F188E; EC67 5B3A E6E3 6A84 9CB2 94D3 BFCD BD57 039F 188E -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Understanding clock drift in virtual machines w/ CFS, NOHZ
Good day. I have come across the occasional issue of clock drift when running (earlier) kernels inside a virtual machine. The reason for drift is simple enough - host may be under heavy load, guest may not get enough resources to run all the ticks (HZ) it is supposed to. At least to put it simply. Now, I have been wondering how the introduction of CFS and CONFIG_NOHZ (dynticks) change this, or if they do at all. If I have understood correctly, only the host's scheduler is really involved so it enters the picture when host is running a recent kernel. In a pure hypervisor-only virtualization, that point should be moot. And then we have dynticks. Try as I might, I haven't been able to wrap my head around the combination. What happens to the kernel and timekeeping when guest has NOHZ enabled? My google-fu is not good enough to find the relevant documentation if such even exists on this subject. To top it off, I'm not certain whether my question is trivial, non-issue or just plain weird. Any pointers on where I should look for more information will be appreciated. -- Mika Boström +358-40-525-7347 -=- The flogging will continue [EMAIL PROTECTED]www.iki.fi/bostik -=- until morale improves GPG: 0x039F188E; EC67 5B3A E6E3 6A84 9CB2 94D3 BFCD BD57 039F 188E -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: the "Turing Attack" (was: Sabotaged PaXtest)
[Posted only on LKML, this has become humour.] On Thu, Feb 10, 2005 at 09:03:00PM +0100, David Weinehall wrote: > On Thu, Feb 10, 2005 at 04:21:49PM +0100, Ingo Molnar wrote: > > > > * Jakob Oestergaard <[EMAIL PROTECTED]> wrote: > > > > PaX cannot be a 'little bit pregnant'. (you might argue that exec-shield > > > > is in the 6th month, but that does not change the fundamental > > > > end-result: a child will be born ;-) > > > > > > Yes and no. I would think that the chances of a child being born are > > > greater if the pregnancy has lasted successfully up until the 6th month, > > > compared to a first week pregnancy. > > > > > > I assume you get my point :) > > > > the important point is: neither PaX nor exec-shield can claim _for sure_ > > that no child will be born, and neither can claim virginity ;-) > > > > [ but i guess there's a point where a bad analogy must stop ;) ] > > Yeah, sex is *usually* a much more pleasant experience than having your > machine broken into, even if it results in a pregnancy. =) I'll bite, before anyone else says it... It can not be a mere coincidence that the most rigorous security audits include penetration testing. -- Mika Boström +358-40-525-7347 \-/ "World peace will be achieved [EMAIL PROTECTED]www.iki.fi/bostik Xwhen the last man has killed Security freak, and proud of it./-\ the second-to-last." -anon? signature.asc Description: Digital signature
Re: the Turing Attack (was: Sabotaged PaXtest)
[Posted only on LKML, this has become humour.] On Thu, Feb 10, 2005 at 09:03:00PM +0100, David Weinehall wrote: On Thu, Feb 10, 2005 at 04:21:49PM +0100, Ingo Molnar wrote: * Jakob Oestergaard [EMAIL PROTECTED] wrote: PaX cannot be a 'little bit pregnant'. (you might argue that exec-shield is in the 6th month, but that does not change the fundamental end-result: a child will be born ;-) Yes and no. I would think that the chances of a child being born are greater if the pregnancy has lasted successfully up until the 6th month, compared to a first week pregnancy. I assume you get my point :) the important point is: neither PaX nor exec-shield can claim _for sure_ that no child will be born, and neither can claim virginity ;-) [ but i guess there's a point where a bad analogy must stop ;) ] Yeah, sex is *usually* a much more pleasant experience than having your machine broken into, even if it results in a pregnancy. =) I'll bite, before anyone else says it... It can not be a mere coincidence that the most rigorous security audits include penetration testing. -- Mika Boström +358-40-525-7347 \-/ World peace will be achieved [EMAIL PROTECTED]www.iki.fi/bostik Xwhen the last man has killed Security freak, and proud of it./-\ the second-to-last. -anon? signature.asc Description: Digital signature
