On Thu, May 31, 2018 at 11:19:04AM -0400, John Ferlan wrote:
On 05/31/2018 08:14 AM, Martin Kletzander wrote:On Wed, May 30, 2018 at 12:00:26PM -0400, John Ferlan wrote:On 05/21/2018 11:00 AM, Martin Kletzander wrote:TSEG (Top of Memory Segment) is one of many regions that SMM (System Management Mode) can occupy. This one, however is special, because a) most of the SMM code lives in TSEG nowadays and b) QEMU just (well, some time ago) added support for so called 'extended' TSEG. The difference to the TSEG implemented in real q35's MCH (Memory Controller Hub) is that it can have any size from 1 MiB up to 65534 MiB in 1 MiB increments. But more about that in the QEMU patch.^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Which some reader, but not this one, may be eager to find ;-) Still is there a valid range QEMU would accept? Or do we just let QEMU fail if the range is too high? I think QEMU has MCH_HOST_BRIDGE_EXT_TSEG_MBYTES_MAXRather than promising some value, I adjusted it so that it is no longer false, no matter what the max is there.Signed-off-by: Martin Kletzander <mkletzan@xxxxxxxxxx> --- docs/formatdomain.html.in | 39 +++++++++++++++++++ docs/schemas/domaincommon.rng | 5 +++ src/conf/domain_conf.c | 60 ++++++++++++++++++++++++++++- src/conf/domain_conf.h | 1 + tests/genericxml2xmlindata/tseg.xml | 23 +++++++++++ tests/genericxml2xmltest.c | 2 + 6 files changed, 129 insertions(+), 1 deletion(-) create mode 100644 tests/genericxml2xmlindata/tseg.xmlIn the category of I hate it when that happens, git am -3 "merged" in two chunks incorrectly! Probably wouldn't have happened if I'd doneYou can enable/disable 3-way merges if you do (not) like them.this sooner! The virDomainDefFeaturesCheckABIStability hunk got merged into virDomainRedirFilterDefCheckABIStability and the tseg grammar got merged under "vmport" and just before "gic". As you can imagine the results weren't pretty ;-).Yeah, happened to me as well, I should've resent this, but I forgot about the merge issue and I also wanted to show that this was posted way before the freeze. Anyway, it's pointless to force it now, I'll leave it for later (meaning "after the release").*Lots* of things were posted after this that got reviewed and pushed before this - that just seems to be "the norm" (for whatever reason)... Not receiving timely reviews is one of those areas that really bothers me. I've been making the effort more recently to try to go in order of longest without review - sometimes a ping makes me lose order though.Anyway, I keep my branches updated (every now and then) on my github repo [1], so if you want to check that, you always can. [1] https://github.com/nertpinx/libvirtOK - I've added that as a remote...diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in index 403b638bd4bd..39ebfe398bd7 100644 --- a/docs/formatdomain.html.in +++ b/docs/formatdomain.html.in @@ -1897,6 +1897,9 @@ <ioapic driver='qemu'/> <hpt resizing='required'/> <vmcoreinfo state='on'/> + <smm state='on'> + <tseg unit='MiB'>48</tseg> + </smm> </features> ...</pre> @@ -2056,6 +2059,42 @@ <code>off</code>, default <code>on</code>) enable or disable System Management Mode. <span class="since">Since 2.1.0</span> + + Optional sub-element <code>tseg</code> can be used to specify the + amount of memory dedicated to SMM TSEG. The size can be specified as a + value of that element, optional attribute <code>unit</code> can be + used to specify the unit of the aforementioned value (defaults to + 'MiB'). +If this is to be a true paragraph break then these paragraphs needs to be wrapped in <p> ... </p>; otherwise, this just becomes one long run on (and quite ugly IMO) paragraph.+ This value is configurable due to the fact that the calculation cannot + be done right with the guarantee that it will work correctly. For + QEMU TSEG was disabled up to and including <code>pc-q35-2.9</code> (it + does not make sense fo any other machine type than q35).s/fo/for/ This also appears to be a paragraph break...+ From <code>pc-q35-2.10</code> the default value was changed to 16 MiB.s/From/Starting with/ ??? (not required, just a though)+ That should suffice for up to 272 VCPUs, 5 GiB guest RAM in total, no + hotplug memory range, and 32 GiB of 64-bit PCI MMIO aperture. Or for + 48 VCPUs, with 1TB of guest RAM, no hotplug DIMM range, and 32GB of + 64-bit PCI MMIO aperture. The values may also vary based on the loader + the VM is using. + + Additional size might be needed for significantly higher VCPU counts + or increased address space (that can be memory, maxMemory, 64-bit PCI + MMIO aperture size; roughly 8 MiB of TSEG per 1 TiB of address space) + which can also be rounded up.Uh, oh, hmmm... We seem to have this (perhaps more recent) "thing" about libvirt having to supply some attribute based on some (mostly difficult to describe) algorithm that QEMU would use in order to create the "optimum" size or use for some alternate algorithm. Of course, a few libvir-list patches like that have been NACK'd because of the feeling that providing a useful algorithm for a user to "decide upon" a satisfactory attribute value cannot really be done. Off the top of my head I can come up with two:It's kind of a different story. Think of this as a memory size. You cannot determine the "right" amount of memory the VM should have. You can try to boot with X and double it until the OS installation succeeds. And hope you won't need to change it later.1. Add poll-max-ns property of each iothread: https://www.redhat.com/archives/libvir-list/2017-February/msg01047.htmlThis is about tunables. It might change the performance/latency of QEMU slightly, but that's about it.and there are those that would find it useful to have (bz 1545732)... If
Much of what I read (underscore emphasis mine) suggests otherwise: - "There is a lot of sentiment *against* providing too many low level knobs like this _without proper guidance_ on how they should be set." - "To address this issue QEMU implements self-tuning algorithm that modifies the current polling time to _adapt to different workloads_ and it can also fallback to blocking syscalls." - "The QEMU commits say the tunables all default to sane parameters so I'm inclined to say we ignore them at the libvirt level entirely." - "I'm fine if libvirt doesn't add a dedicated API for setting <iothread> polling parameters. It's _unlikely_ that users will need to change the setting. In an emergency (e.g. disabling it due to a performance regression) _they can_ use <qemu:arg value='-newarg'/>." The only points for the polling to be enabled were along the lines of: It _may_ help in _some_ workloads when you want a bit more throughput for the price of more CPU cycles. With vague definitions of how much CPU, throughput and without description of how to find out if a particular workload fits this. Even when all of that is there, then you need yet another explanation on how to calculate the value to be set. And then it all goes down back to the fact that QEMU is already doing some automated balancing for this (because they can, because this is not part of the guest ABI). That way you can never actually say if it will help and how much. So for this one it is a clear "NO".
you don't have enough memory and your VM is paging like crazy, you just add more memory. Requires a reboot. Likewise, if your VM doesn't boot you add/alter the magic TSEG value using some algorithm as described above. From a 90,000 foot customer view is there a difference? It's just a knob that the hypervisor has to allow something to be accomplished for which libvirt provides the attribute to fine tune.
Yeah, you're right. That's why I think both of them should be exposed. Some small differences to other knobs, just for completeness: - by the time you realize that the VM doesn't have enough memory, it might be too late as reboot isn't that easy of a thing for some production workloads - on the other hand, you have a way to see that happening (compare it to the polling interval above which you have no idea without proper benchmarks) One more thing that's common to the memory size (and I hope TSEG in the future) is that in mgmt apps the TSEG setting already has a place where to live and it is exactly where the memory size lives currently. In templates. You have "small vm" teplate and "ginormous vm". For the latter one you can just add a setting of TSEG _once_ per file. How's TSEG better and easier than memory? You figure it once for the VM settings (and possibly firmware, but that's not going to change much) and then it doesn't depend on the workflow, not even a little bit. Anyway. What I see as the differences between tunables that make it in and tunables that don't is that: - the former are usually understandable and easy to see what they are in bare metal. Everyone knows what memory is in the hardware, how it looks like, how much is "not enough" and how much is "more that needed". We are used to those things back from the hardware times, even to changing them. - The latter is usually something we were not able control in HW or didn't even know it existed. For virtual workloads it might be completely different, but sometimes people are forgetting that.
2. Add support for qcow2 cache (many times, but most recently): https://www.redhat.com/archives/libvir-list/2017-September/msg00553.htmlSimilarly here, it allows setting something that can be (at least slightly) abstracted and in the worst case the performance will be slightly hindered.This one I understand more why it would be rejected, but still providing the value allows certain things to work a whole lot better. I also know Berto has been "fine tuning" the algorithm in later QEMU releases - so that's like hitting a moving target.
This is very similar, it's just that there is no automatic balancing
done by QEMU. But it usually is also about how you write the docs. The
option can make very much sense, but if someone writes "Setting asdf can
allows fine-tuning of the asdf value in the underlying hypervisor", then
no matter how much that value makes sense it is not reflected in the
docs. That's why I tried to add all the relevant info into the docs so
that it's clear what it is doing, how to set it, to what values and
when.
Apart from the fact that there is a "link" to some file in the QEMU
repository that someone is supposed to read, plus the decision for the
value determination are written there (but not why they are not
automatically calculated, or maybe I missed it), it:
- is not possible to try using the <qemu:arg value='-newarg'/> approach
- the docs say:
<b>In general you should leave this option alone, unless you
are very certain you know what you are doing.</b>
So in this particular case I wouldn't be totally against having it
there. If you don't want to use it, then "just don't touch that" is an
approach that shouldn't hurt anyone.
Contrast this with TSEG which, in case it is set incorrectly, will prevent the machine from booting at all. If we go to the extreme, not only can you easily try to find out the right amount to set for a particular machine, but you can even do that programmatically since when OVMF fails due to small extended TSEG size it will reboot very fast. And you can get that in form of events. When I tried it now it even looks like you get rtc-change event when the domain doesn't reboot immediately due to small TSEG size.Is there something that is provided that would give the "hint" to adjust the TSEG size?
I thought there was, but maybe I'll add some more info after the discussion.
I will not put it in the docs because I will not guarantee that this is the right way to go, but this is how events look for default TSEG size for a guest that needs a lot more (it has 240 possible vCPUs and 256 TiB of maximum memory, but because it starts with only 1 vCPU and 1GiB memory I can try it out easily on my machine: virsh # event --domain nixos --all --loop --timestamp 2018-05-31 09:42:21.060+0000: event 'lifecycle' for domain nixos: Resumed Unpaused 2018-05-31 09:42:21.066+0000: event 'lifecycle' for domain nixos: Started Booted 2018-05-31 09:42:21.514+0000: event 'reboot' for domain nixos 2018-05-31 09:42:21.964+0000: event 'reboot' for domain nixos 2018-05-31 09:42:22.414+0000: event 'reboot' for domain nixos 2018-05-31 09:42:22.868+0000: event 'reboot' for domain nixos 2018-05-31 09:42:23.325+0000: event 'reboot' for domain nixos 2018-05-31 09:42:23.778+0000: event 'reboot' for domain nixos 2018-05-31 09:42:24.230+0000: event 'reboot' for domain nixos 2018-05-31 09:42:24.681+0000: event 'reboot' for domain nixos ... you get the point. And this is how it looks when I start it with increased size: virsh # event --domain nixos --all --loop --timestamp 2018-05-31 09:43:24.578+0000: event 'lifecycle' for domain nixos: Resumed Unpaused 2018-05-31 09:43:24.584+0000: event 'lifecycle' for domain nixos: Started Booted 2018-05-31 09:43:31.808+0000: event 'rtc-change' for domain nixos: 0 2018-05-31 09:43:32.808+0000: event 'rtc-change' for domain nixos: 0 The reasons for this not being done automatically are (from the top of my head): - The above is just something I figured out myself, but it's not the recommended way written anywhere. Maybe I'm wrong and it doesn't really work, but it still can be done manually. - You cannot change it however you would like automatically, it is part of the guest ABI and we are striving for keeping that stable. - Trying to figure this out by 1 MiB increments might take some time, but increasing it faster might be wasteful. Basically there is no one-size-fits-all value, no easy way to do it automatically (maybe what I tried), but very good explanation how to do that manually and very easy way to do that. Also, from the SW POV, it doesn't even depend on the guest OS, just on the loader/bios so if you have two same domains (like a template in OpenStack for example) you try it out once and then you have the value that just works and will continue working until you change something for the domain. And what it depends on is clearly written in the documentation.And there are those that could say if the underlying hypervisor knows that for certain memory sizes and/or vCPU counts that the TSEG will be too small for specific machine types that then the underlying hypervisor should be the one to "choose" a value that's programatically appropriate which to a degree IIUC is the argument being used against allowing a libvirt knob for the poll-max-ns and qcow2 cache sizes.
And they would be wrong as for TSEG the hypervisor a) doesn't know that and b) cannot change that once it was started.
+ + Due to the nature of this setting being similar to "how much RAM + should the guest have" users are advised to either consult the + documentation of the guest OS or loader (if there is any), or test + this by trial-and-error changing the value until the VM boots + successfully. Yet another guiding value for users might be the fact + that 48 MiB should be enough for pretty large guests (240 VCPUs and + 4TB guest RAM), but it is on purpose not set as default as 48 MiB of + unavailable RAM might be too much for small guests (e.g. with 512 MiB + of RAM).and this is the exact reason why patches like this get NACKd - because trial and error should not be a 'desired' means to calculate.It is not. They are rejected because either a) there is no documentation how to properly check if the value is the right value when doing trial-end-error (this is not the case here since you can see if the machine boots or not) or b) the values being set are too specific instead of being abstracted -- setting value in KiB between 0 and the size of a disk instead of "max_performance" or "min_latency" (this is not the case here, the documentation explains what the size is and why it is not about few guessable values). Basically we are NACKing simple pass-through values without understanding them and adding some documentation for them. For example stuff for which we have documentation along the lines of: "Element asdf can be used to set the asdf of the domain."I guess it's all a matter of how things are being viewed. I can certainly see how someone without a redhat.com email may view this series as similar to the referenced ones. It's more about mitigating failure possibilities by being able to provide knobs that can help (or get you into trouble). Not everyone has large configurations - so do they really need the value? Not everyone needs every knob, but having them available for when you do need them isn't a bad thing.bz referenced in patch 5 has an incredible amount of data and calculations that provide even more insight and details that are lost when we try to summarize in a libvirt meaningful patch.Let me know what relevant information from the bz you are missing in the documentation and I'll gladly add it.If you think what's provided above is enough, then leave it as is. If there's something in that bz or in the ensuing discussion, then add it. Even though it's difficult when you're in the middle of the code - if you were someone who ran into this situation without knowing anything about it - would what's provided above give enough information to help you decide which value to use? That whole trial and error comment is what leaves me wondering as a reader who knew nothing about this before I started. And now I have more I need a pub to help forget ;-)
Here comes the weekend!
What it seems is really needed is an attribute that libvirt provides that tells QEMU to calculate the optimum size.+ + See <a href="#elementsMemoryAllocation">Memory Allocation</a> + for more details about the <code>unit</code> attribute. + <span class="since">Since 4.5.0</span> (QEMU only)haha - see you put 4.5.0 and this is the 4.4.0 release - so it was ignored until 4.5.0 was "on the clock" ;-) Ironically this is pointed out as QEMU only; however, genericxml2xmltest is used/updated.So?Why would someone add this generically if it only works for QEMU?
The part where the VM is set up based on that is only supported in QEMU. The part that does formatting/parsing is done in src/conf/domain_conf.c, which is used by all drivers. I would add it there because it: a) might be later added to other hypervisor drivers b) even when someone is compiling with the qemu driver turned off they cannot break it (provided they do run tests)
So, I personally don't mind if this attribute is added; however, I think we become hypocrites to a certain degree if patches continue to be blocked/NACKed to other subsystems that have attributes that allow certain amount of control, but don't come with exact sizing references. Still if this is pushed, then perhaps those others can use this as the means to provide a reference to other knobs added.How much more exact would you wanted to be in terms of sizing? If it could be any more exact we wouldn't need the tunable at all. Please don't compare it to other tunables that we didn't want exposed just because it "sounds similar". I hear lot of people just put stuff like this into "unknown knobs" box and they treat it the same. But there are differences and it's not allYou can have my : Reviewed-by: John Ferlan <jferlan@xxxxxxxxxx>I wasn't ever against the R-b, but I guess I'm missing the point. You disagree with me on the change made here, but then I get a R-b? :) And then I go further down and read that the R-b has actually no value at all and I should wait for another one :D Maybe I'm overthinking this, but it didn't used to happen back when we used ACKs :)with a few adjustments above; however, another R-By should be obtained here as well as perhaps a policy change so that other similar such series could be merged... I guess I'm curious what "thoughts" others may have regarding adding this "knob" while not allowing others. JohnDid I disagree on the change or was I pointing out that from a certain abstraction level it could be said the code is "similar to" the patches I referenced w/r/t libvirt supplying some value that perhaps the underlying hypervisor could/should have adjusted instead based on it's knowledge of the rather involved algorithms. FWIW: I'm not opposed to the referenced patches, but I'm in the minority on them. AIUI, the R-b is essentially stating that I've looked at the patch and in general agree with it. As part of that - there are a few things that need adjustment and since I felt that it had some similarities to the referenced patches w/r/t adding a knob that some may feel isn't necessary I said let's make sure there's "enough consensus" that this should be added (which it seems there is). IIRC, the following was referenced at some point during the R-b discussion: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?h=v4.13#n560 John
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