On Sun, Nov 20, 2016 at 8:45 AM, Harry Putnam <[email protected]> wrote: > "J. Roeleveld" <[email protected]> writes: > >> Also, overcommitting CPUs has a bad influence on performance, >> especially if the host wants to use all cores as well. > > That is what I asked advice about. What do you call > `overcommitting'. For example with only 1 Vbox vm started and no > serious work being done by the windwos-10 os. On an HP xw8600 with > older 2x Xeon 5.60 3.00Ghz with 32 GB ram >
IMO over-committing CPU isn't actually THAT bad. The CPU obviously gets divided n ways, but that's as far as it goes. There isn't that much overhead switching between VMs (though there certainly is some). Over-committing RAM on the other hand can definitely cause more serious issues, because then you're dealing with swap. Dividing 1 CPU 3 ways gives you 1/3rd of a CPU (but collectively the 3 VMs are putting out close to a full CPU's worth of work). If you're over-committing RAM and you go into swap, then the performance of all your hosts might drop considerably, adding up to WAY less than the total your box is capable of. If your host is windows then this isn't an option for you (seriously, you should re-consider that), but if you could use a linux host another solution is containers. In general they are FAR more flexible around RAM use and of course RAM tends to be the most precious commodity when you're running guests of any kind. With a container you don't have to pre-allocate the RAM, so if Gentoo needs 8GB of RAM right now it isn't as big a deal because in 15min when you're done building it will go back down to 100MB or whatever it actually needs to run. In general Gentoo doesn't need a lot of RAM to operate, it is a fairly minimal distro in general. Now, obviously if you're running webkit then you're running it as more of a desktop and that is going to consume a lot more RAM than a console-based system, on any distro. If anything you could still tweak USE flags and CFLAGS to reduce your RAM footprint compared to most distros (whether this is worthwhile is another matter). However, the one exception to this is when you're building things. Ideally when you're building you want to use lots of cores, which means more gcc instances using more RAM each, and you want to be doing all of this on a tmpfs that uses even more RAM. Back when I was running Gentoo VMs I would typically set the RAM use to something fairly minimal (think ~1GB or less). Then when I was doing updates I'd up the setting to basically all the free RAM on my host and allocate multiple CPU cores to it, then mount a tmpfs on /var/tmp. When I was done building I'd shrink it back down to a normal config. And I wouldn't be doing builds on multiple hosts at once. These days with containers I just run emerge on a few at a time and I don't worry about it (still with /var/tmp on a tmpfs in each). Now, I wouldn't go building chromium and libreoffice in multiple containers at once that way, but for typical server-like guests very few packages use THAT much RAM. -- Rich
