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Top Forums UNIX for Advanced & Expert Users Linux Virtual Memory Question Post 302561265 by Corona688 on Monday 3rd of October 2011 05:38:47 PM
Old 10-03-2011
Quote:
Originally Posted by achenle
Linux doesn't have "virtual swap", but by default Linux does overcommit swap space - IMO a REALLY, REALLY BAD IDEA.
I've heard it referred to as virtual swap, and took it as another term for it...

A gridlocked system with insufficient RAM for kill -9 rameater is a tough nut to crack too, with the same fix as the oom-killer: "Don't let it happen". That's why there's programmable limits for these things.

I'll note that Linux systems tend to be poorly configured for swap in general, it's mostly considered a 'quick fix' for brief low-ram/high load conditions... It gets thrown on the same spindle and forgotten. So if it gets used at all, performance truly goes down the toilet.
Quote:
That wouldn't usually need swap space because the backing store for the virtual memory would be your hard disk you mmap()'d, but you could probably map it in such a way that if you were to write to it, the changed pages would require swap backing. Maybe by mmap()'ing a read-only file descriptor read/write?
MAP_PRIVATE ought to do it.
 

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SYSTEMD-DETECT-VIRT(1)						systemd-detect-virt					    SYSTEMD-DETECT-VIRT(1)

NAME
systemd-detect-virt - Detect execution in a virtualized environment SYNOPSIS
systemd-detect-virt [OPTIONS...] DESCRIPTION
systemd-detect-virt detects execution in a virtualized environment. It identifies the virtualization technology and can distinguish full machine virtualization from container virtualization. systemd-detect-virt exits with a return value of 0 (success) if a virtualization technology is detected, and non-zero (error) otherwise. By default, any type of virtualization is detected, and the options --container and --vm can be used to limit what types of virtualization are detected. When executed without --quiet will print a short identifier for the detected virtualization technology. The following technologies are currently identified: Table 1. Known virtualization technologies (both VM, i.e. full hardware virtualization, and container, i.e. shared kernel virtualization) +----------+----------------+--------------------------------------+ |Type | ID | Product | +----------+----------------+--------------------------------------+ |VM | qemu | QEMU software virtualization, | | | | without KVM | | +----------------+--------------------------------------+ | | kvm | Linux KVM kernel virtual machine, | | | | with whatever software, except | | | | Oracle Virtualbox | | +----------------+--------------------------------------+ | | zvm | s390 z/VM | | +----------------+--------------------------------------+ | | vmware | VMware Workstation or Server, and | | | | related products | | +----------------+--------------------------------------+ | | microsoft | Hyper-V, also known as Viridian or | | | | Windows Server Virtualization | | +----------------+--------------------------------------+ | | oracle | Oracle VM VirtualBox (historically | | | | marketed by innotek and Sun | | | | Microsystems), | | | | for legacy and KVM | | | | hypervisor | | +----------------+--------------------------------------+ | | xen | Xen hypervisor (only domU, not dom0) | | +----------------+--------------------------------------+ | | bochs | Bochs Emulator | | +----------------+--------------------------------------+ | | uml | User-mode Linux | | +----------------+--------------------------------------+ | | parallels | Parallels Desktop, Parallels Server | | +----------------+--------------------------------------+ | | bhyve | bhyve, FreeBSD hypervisor | +----------+----------------+--------------------------------------+ |Container | openvz | OpenVZ/Virtuozzo | | +----------------+--------------------------------------+ | | lxc | Linux container implementation by | | | | LXC | | +----------------+--------------------------------------+ | | lxc-libvirt | Linux container implementation by | | | | libvirt | | +----------------+--------------------------------------+ | | systemd-nspawn | systemd's minimal container | | | | implementation, see systemd- | | | | nspawn(1) | | +----------------+--------------------------------------+ | | docker | Docker container manager | | +----------------+--------------------------------------+ | | rkt | rkt app container runtime | +----------+----------------+--------------------------------------+ If multiple virtualization solutions are used, only the "innermost" is detected and identified. That means if both machine and container virtualization are used in conjunction, only the latter will be identified (unless --vm is passed). OPTIONS
The following options are understood: -c, --container Only detects container virtualization (i.e. shared kernel virtualization). -v, --vm Only detects hardware virtualization). -r, --chroot Detect whether invoked in a chroot(2) environment. In this mode, no output is written, but the return value indicates whether the process was invoked in a chroot() environment or not. --private-users Detect whether invoked in a user namespace. In this mode, no output is written, but the return value indicates whether the process was invoked inside of a user namespace or not. See user_namespaces(7) for more information. -q, --quiet Suppress output of the virtualization technology identifier. -h, --help Print a short help text and exit. --version Print a short version string and exit. EXIT STATUS
If a virtualization technology is detected, 0 is returned, a non-zero code otherwise. SEE ALSO
systemd(1), systemd-nspawn(1), chroot(2), namespaces(7) systemd 237 SYSTEMD-DETECT-VIRT(1)
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