[Bug 199727] CPU freezes in KVM guests during high IO load on host

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=199727

--- Comment #4 from Roland Kletzing (devzero@web.de) ---
http://blog.vmsplice.net/2015/08/asynchronous-file-io-on-linux-plus-ca.html

"However, the io_submit(2) system call remains a treacherous ally in the quest
for asynchronous file I/O. I don't think much has changed since 2009 in making
Linux AIO the best asynchronous file I/O mechanism.

The main problem is that io_submit(2) waits for I/O in some cases. It can
block! This defeats the purpose of asynchronous file I/O because the caller is
stuck until the system call completes. If called from a program's event loop,
the program becomes unresponsive until the system call returns. But even if
io_submit(2) is invoked from a dedicated thread where blocking doesn't matter,
latency is introduced to any further I/O requests submitted in the same
io_submit(2) call.

Sources of blocking in io_submit(2) depend on the file system and block devices
being used. There are many different cases but in general they occur because
file I/O code paths contain synchronous I/O (for metadata I/O or page cache
write-out) as well as locks/waiting (for serializing operations). This is why
the io_submit(2) system call can be held up while submitting a request.

This means io_submit(2) works best on fully-allocated files, volumes, or block
devices. Anything else is likely to result in blocking behavior and cause poor
performance."

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