On Mon, Sep 04, 2017 at 04:14:00PM +0200, Martin Kletzander wrote:
* The current design (finally something libvirt-related, right?)
The discussion ended with a conclusion of the following (with my best knowledge, there were so many discussions about so many things that I would spend too much time looking up all of them):
- Users should not need to specify bit masks, such complexity should be abstracted. We'll use sizes (e.g. 4MB)
- Multiple vCPUs might need to share the same allocation.
- Exclusivity of allocations is to be assumed, that is only unoccupied cache should be used for new allocations.
The last point seems trivial but it's actually very specific condition that, if removed, can cause several problems. If it's hard to grasp the last point together with the second one, you're on the right track. If not, then I'll try to make a point for why the last point should be removed in 3... 2... 1...
* Design flaws
1) Users cannot specify any allocation that would share only part with some other allocation of the domain or the default group.
2) It was not specified what to do with the default resource group. There might be several ways to approach this, with varying pros and cons:
a) Treat it as any other group. That is any bit set for this group will be excluded from usable bits when creating new allocation for a domain.
- Very predictable behaviour
- You will not be able to allocate any amount of cache without previous setting for the default group as that will have all the bits set which will make all the cache unusable
b) Automatically remove the appropriate amount of bits that are needed for new domains.
- No need to do any change to the system settings in order to use this new feature
- We would have to change system settings, which is generally frowned upon when done "automatically" as a side effect of starting a domain, especially for such scarce resource as cache
- The change to system settings would not be entirely predictable
c) Act like it doesn't exist and don't remove its allocations from consideration
- Doesn't really make sense as system processes might be trashing the cache as any VM, moreover when all VM processes without allocations will be based in the default group as well
3) There is no way for users to know what the particular settings are for any running domain.
The first point was deemed a corner case. Fair enough on its own, but considering point 2 and its solutions, it is rather difficult for me to justify it. Also, let's say you have domain with 4 vCPUs out of which you know 1 might be trashing the cache, but you don't want to restrict it completely, but others will utilize it very nicely. Sensible allocations for such domain's vCPUs might be:
vCPU 0: 000f vCPUs 1-3: ffff
as you want vCPUs 1-3 to utilize even the part of cache that might get trashed by vCPU 0. Or they might share some data (especially guest-memory-related).
The case above is not possible to set up with only per-vcpu(s) scalar setting. And there are more as you might imagine now. For example how do we behave with iothreads and emulator threads?
Ok, I see what you're getting at. I've actually forgotten what our current design looks like though :-) What level of granularity were we allowing within a guest ? All vCPUs use separate cache regions from each other, or all vCPUs use a share cached region, but separate from other guests, or a mix ?
* My suggestion:
- Provide an API for querying and changing the allocation of the default resource group. This would be similar to setting and querying hugepage allocations (see virsh's freepages/allocpages commands).
Reasonable
- Let users specify the starting position in addition to the size, i.e. not only specifying "size", but also "from". If "from" is not specified, the whole allocation must be exclusive. If "from" is specified it will be set without checking for collisions. The latter needs them to query the system or know what settings are applied (this should be the case all the time), but is better then adding non-specific and/or meaningless exclusivity settings (how do you specify part-exclusivity of the cache as in the example above)
I'm concerned about the idea of not checking 'from' for collisions, if there's allowed a mix of guests with & within 'from'. eg consider * Initially 24 MB of cache is free, starting at 8MB * run guest A from=8M, size=8M * run guest B size=8M => libvirt sets from=16M, so doesn't clash with A * stop guest A * run guest C size=8M => libvirt sets from=8M, so doesn't clash with B * restart guest A => now clashes with guest C, whereas if you had left guest A running, then C would have got from=24MB and avoided clash IOW, if we're to allow users to set 'from', I think we need to have an explicit flag to indicate whether this is an exclusive or shared allocation. That way guest A would set 'exclusive', and so at least see an error when it got a clash with guest C in the example.
- After starting a domain, fill in any missing information about the allocation (I'm generalizing here, but fro now it would only be the optional "from" attribute)
- Add settings not only for vCPUs, but also for other threads as we do with pinning, schedulers, etc.
Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|