| OpenVZ / LXC / Zones | KVM | |
|---|---|---|
| CPU Performance | native | high (with CPU support) |
| CPU Allocation | flexible (FSS + “bursting”) | fixed to VCPU limit |
| I/O Throughput | high (no overhead) | low or medium (with virtio) |
| I/O Latency | low (native) | some (I/O proxy overhead) |
| Memory Access Overhead | none | some (EPT/NPT or shadow page tables) |
| Memory Loss | none | some (extra kernels; page tables) |
| Memory Allocation | flexible (unused memory used for file system cache) | fixed (and possible double-caching) |
| Resource Controls | many (depends on OS) | most (OS + hypervisor) |
| Observability from the host | highest (see everything) | medium (resource usage, hypervisor statistics, OS inspection of hypervisor) |
| Observability: from the guest | medium (see everything permitted, incl. some physical resource stats) | low (guest only) |
| Hypervisor Complexity | low (OS partitions) | medium |
| Different OS Guests | no | yes |
Networking (Container vs KVM) aprox. 5x slower.
HDD IO (Container vs KVM) aprox. 4x slower.
/* * GUEST # QEMU * ##################################################################### * # * +----------+ # * | start_ | (1) # * | xmit() | # * +----------+ # * || # * || +-----------+ # * ||------>|free_old_ | (2) # * ||------>|xmit_skbs()| # * || +-----------+ # * \/ (3) # * +---------+ +-------------+ + - #--- PIO write to VNIC * | xmit_ |------->|virtqueue_add| | # PCI config space (6) * | skb() |------->|_buf_gfp() | | # * +---------+ +-------------+ | # * || | # +- VM exit * || +- iff interrupts | # | KVM driver exit (7) * \/ | unmasked (4) | # | * +---------+ | +-----------+(5) | # | +---------+ * |virtqueue|----*---->|vp_notify()|-----*---#-*->| handle | (8) * |_kick() |----*---->| |-----*---#-*->|PIO write| * +---------+ +-----------+ # +---------+ * || # || * || (13) # || * **-----+ iff avail ring # \/ (9) * || capacity < 20 # +-----------------+ * || else return # |virtio_net_handle| * || # |tx_timer() | * \/ (14) # +-----------------+ * +----------+ # || * |netif_stop| # || (10) * |_queue() | # || +---------+ * +----------+ # ||-->|qemu_mod_| * || # ||-->|timer() | * || (15) (16) # || +---------+ * +----------------+ +----------+ # || * |virtqueue_enable|---->|unmask | # || (11) * |_cb_delayed() |---->|interrupts| # || +------------+ * +----------------+ +----------+ # |+->|virtio_ | * || || # +-->|queue_set_ | * || (18) || (17) # |notification| * || +-return +-------------------+ # +------------+ * || | iff ---->|check if the number| # | * **--+ is false |of unprocessed used| # | disable host * || |ring entries is > | # +- interrupts * || |3/4s of the avail | # (12) * \/ (19) |ring index - the | # * +-----------+ |last freed used | # * |free_old_ | |ring index | # * |xmit_skbs()| +-------------------+ # * +-----------+ # * || # * || (20) # * **-----+ iff avail ring # * || capacity is # * || now > 20 # * \/ # * +-----------+ # * |netif_start| (21) # * |_queue() | # * +-----------+ # * || # * || # * \/ (22) (23) # * +------------+ +----------+ # * |virtqueue_ |----->|mask | # * |disable_cb()|----->|interrupts| # * +------------+ +----------+ # * # * # */
Figure II: Guest / Host Packet TX Part 1</code>