Daniel P. Berrangé <[email protected]> wrote: > On Fri, Jan 03, 2020 at 07:25:08PM +0100, Juan Quintela wrote: >> Daniel P. Berrangé <[email protected]> wrote: >> > On Wed, Dec 18, 2019 at 03:01:10AM +0100, Juan Quintela wrote: >> >> We can scale much better with 16, so we can scale to higher numbers. >> > >> > What was the test scenario showing such scaling ? >> >> On my test hardware, with 2 channels we can saturate around 8Gigabit max, >> more than that, and the migration thread is not fast enough to fill the >> network bandwidth. >> >> With 8 that is enough to fill whatever we can find. >> We used to have a bug where we were getting trouble with more channels >> than cores. That was the initial reason why the default was so low. >> >> So, pros/cons are: >> - have low value (2). We are backwards compatible, but we are not using >> all bandwith. Notice that we will dectect the error before 5.0 is >> out and print a good error message. >> >> - have high value (I tested 8 and 16). Found no performance loss when >> moving to lower bandwidth limits, and clearly we were able to saturate >> the higher speeds (I tested on localhost, so I had big enough bandwidth) >> >> >> > In the real world I'm sceptical that virt hosts will have >> > 16 otherwise idle CPU cores available that are permissible >> > to use for migration, or indeed whether they'll have network >> > bandwidth available to allow 16 cores to saturate the link. >> >> The problem here is that if you have such a host, and you want to have >> high speed migration, you need to configure it. My measumermets are >> that high number of channels don't affect performance with low >> bandwidth, but low number of channels affect performance with high >> bandwidth speed. > > I'm not concerned about impact on performance of migration on a > low bandwidth link, rather I'm concerned about impact on performance > of other guests on the host. It will cause migration to contend with > other guest's vCPUs and network traffic.
Two things here: - vcpus: If you want migration to consume all the bandwidth, you are happy with it using more vcpus. - bandwidth: It will only consume only the one that the guest has assigned, split (we hope evenly) between all the channels. My main reason to have a higher number of channels is: - test better the code with more than one channel - work "magically" well in all scenarios. With a low number of channels, we are not going to be able to saturate a big network pipe. > >> So, if we want to have something that works "automatically" everywhere, >> we need to put it to at least 8. Or we can trust that management app >> will do the right thing. > > Aren't we still setting the bandwidth limit to MB bandwidth out of the > box, so we already require mgmt app to change settings to use more > bandwidth ? Yeap. This is the default bandwidth. #define MAX_THROTTLE (32 << 20) >> If you are using a low value of bandwidth, the only difference with 16 >> channels is that you are using a bit more memory (just the space for the >> stacks) and that you are having less contention for the locks (but with >> low bandwidth you are not having contention anyways). >> >> So, I think that the question is: Note that my idea is to make multifd "default" in the near future (5.1 timeframe or so). >> - What does libvirt prefferes > > Libvirt doesn't really have an opinion in this case. I believe we'll > always set the number of channels on both src & dst, so we don't > see the defaults. What does libvirt does today for this value? >> - What does ovirt/openstack preffer > > Libvirt should insulate them from any change in defaults in QEMU > in this case, but always explicitly setting channels on src & dst > to match. I agree here, they should don't care by default. >> - Do we really want that the user "have" to configure that value > > Right so this is the key quesiton - for a user not using libvirt > or a libvirt based mgmt app, what we do want out out of the box > migration to be tuned for ? In my opinion, we should have something like: - multifd: enabled by default - max downtime: 300 ms (current) looks right to me - max bandwidth: 32MB/s (current) seems a bit low. 100MB/s (i.e. almost full gigabit ethernet) seems reasonable to me. Having a default for 10Gigabit ethernet or similar seems too high. > If we want to maximise migration performance, at cost of anything > else, then we can change the migration channels count, but probably > also ought to remove the 32MB bandwidth cap as no useful guest with > active apps will succeed migration with a 32MB cap. Will start another series with the current values to discuss all the defaults, ok? thanks for the comments, Juan.
