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Re: [Xen-devel] OOM problems

They are throttled, but the single control I'm aware of
is /proc/sys/vm/dirty_ratio (or dirty_bytes, nowadays). Which is only
per process, not a global limit. Could well be that's part of the
problem -- outwitting mm with just too many writers on too many cores?

We had a bit of trouble when switching dom0 to 2.6.32, buffered writes
made it much easier than with e.g. 2.6.27 to drive everybody else into
costly reclaims.

The Oom shown here reports about ~650M in dirty pages. The fact alone
that this counts as on oom condition doesn't sound quite right in
itself. That qemu might just have dared to ask at the wrong point in

Just to get an idea -- how many guests did this box carry?

It carries about two dozen guests, with a mix of mostly HVMs (all stubdom-based, some with PV-on-HVM drivers) and some PV.

This problem occurred more often for me under 2.6.32 than 2.6.31, I noticed. Since I made the switch to aio, I haven't seen a crash, but it hasn't been long enough for that to mean much.

Having extra caching in the dom0 is nice because it allows for domUs to get away with having small amounts of free memory, while still having very good (much faster than hardware) write performance. If you have a large number of domUs that are all memory-constrained and use the disk in infrequent, large bursts, this can work out pretty well, since the big communal pool provides a better value proposition than giving each domU a few more megabytes of RAM.

If the OOM problem isn't something that can be fixed, it might be a good idea to print out a warning to the user when a domain using "file:" is started. Or, to go a step further and automatically run "file" based domains as though "aio" was specified, possibly with a warning and a way to override that behavior. It's not really intuitive that "file" would cause crashes.


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