Hello, This question is frequently posed by those new to Linux: "Where has all my ram gone?!"
The answer is disk caching! I'd like to do some testing where I'll be firing up about 7 virtual machines on my workstation. I'm using qemu+kvm, which does a good job about not grabbing ram until it is used. That way, I can fire up 7 VMs, each allowed to use up to 512 MB of ram. I'm still below my actual physical ram capacity of 4GB, but I'd like to still have use of any ram that I can on the host. As long as my VMs don't allocate any memory, I should be fine. The disk caching feature of Linux will guarantee that over time, with use, each VM will eventually make full use of the 512MB of ram. If there's a way to disable that feature in Linux altogether, I can have some spare ram on the host at the cost of performance of the guest VMs which is acceptable in my situation. I've found the /proc/sys/vm/drop_caches location, but I'm not clear on whether this is a one-shot drop, or if it'll actually disable caching. If it's a one shot drop, it isn't terribly useful, but will help some if I drop once a minute with cron. If needed, I'm happy to recompile the kernel, but I'd rather not have to. If I'm overlooking another feature of qemu-kvm that'd make my life easier, I'd love to know about it. I do believe that since each VM is running the same version of Linux, that kvm can do some shared memory magic, but I don't know if I have to explicitly request that feature. Thanks! Jeff Anderson
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