native Linux AIO support revisited

This patchset introduces support native Linux AIO. The first patch just refactors the existing AIO emulation by thread pools to have a cleaner layering which allows the native AIO support to be implemented more easily.

The second patch introduces real native Linux AIO support, although due to limitations in the kernel implementation we only can use it for cache=none. It is vaguely based on Anhony's earlier patches, but due to the refactoring in the first patch is is much simpler. Instead of trying to fit into the model of the Posix AIO API we directly integrate into the raw-posix code with a very lean interface (see the first patch for a more detailed explanation). That also means we can just register the AIO completion eventd directly with the qemu poll handler instead of needing an additional indirection.

The IO code performs slightly better than the thread pool on most workloads I've thrown at it, and uses a lot less CPU time for it:

iozone -s 1024m -r $num -I -f /dev/sdb

output is in Kb/s:

write 16k read 16k write 64k read 64k write 256k read 256k
native 39133 75462 100980 156169 133642 168343

qemu        29998     48334      79870    116393      133090     161360
qemu+aio    32151     52902      82513    123893      133767     164113

dd if=/dev/zero of=$dev bs=20M oflag=direct count=400 dd if=$dev of=/dev/zero bs=20M iflag=direct count=400

output is in MB/s:

write read

native        116   123
qemu          116   100
qemu+aio      116   121

For all of this the AIO code used significantly less CPU time (no coparism to native due to VM startup overhead and other issues)

real user sys

qemu      25m45.885s  1m36.422s  1m49.394s
qemu+aio  25m36.950s  1m14.178s  1m13.179s

Note that the results have quite a bit of varions per run, so qemu+aio beeing faster in one of the tests above shouldn't mean too much, it's also been minimally slower in some. From various runs I would say that for larger block sizes we meat native performance, a little bit sooner with AIO, and a little bit without.

All thes results are on a raw host device and using virtio. With image files on a filesystems there are potential blocking points in the AIO implementation. Those are relatively small or non-existant on already allocated (and at least for XFS that includes preallocated) files, but for spares files including waiting for disk I/O during allocations and need to be avoided to not kill performance. All the results also already include the MSI support for virtio-blk, btw.

Based on this I would recommend to include this patch, but not use it by default for now. After some testing I would suggest to enable it by default for host devices and investigate a way to make it easily usable for files, possibly including some kernel support to tell us which files are "safe".

These patches require my patch to my pthreads mandatory applies first, which already is in Anthony's queue. If you want to use them with qemu-kvm you also need to backout the compatfd changed to raw-block.c first.