On Tue, Mar 19, 2024 at 06:41:28PM +0100, Kevin Wolf wrote: > Am 19.03.2024 um 18:10 hat Daniel P. Berrangé geschrieben: > > On Tue, Mar 19, 2024 at 05:54:38PM +0100, Kevin Wolf wrote: > > > Am 19.03.2024 um 14:43 hat Daniel P. Berrangé geschrieben: > > > > On Mon, Mar 18, 2024 at 02:34:29PM -0400, Stefan Hajnoczi wrote: > > > > > The coroutine pool implementation can hit the Linux vm.max_map_count > > > > > limit, causing QEMU to abort with "failed to allocate memory for > > > > > stack" > > > > > or "failed to set up stack guard page" during coroutine creation. > > > > > > > > > > This happens because per-thread pools can grow to tens of thousands of > > > > > coroutines. Each coroutine causes 2 virtual memory areas to be > > > > > created. > > > > > > > > This sounds quite alarming. What usage scenario is justified in > > > > creating so many coroutines? > > > > > > Basically we try to allow pooling coroutines for as many requests as > > > there can be in flight at the same time. That is, adding a virtio-blk > > > device increases the maximum pool size by num_queues * queue_size. If > > > you have a guest with many CPUs, the default num_queues is relatively > > > large (the bug referenced by Stefan had 64), and queue_size is 256 by > > > default. That's 16k potential requests in flight per disk. > > > > If we have more than 1 virtio-blk device, does that scale up the max > > coroutines too ? > > > > eg would 32 virtio-blks devices imply 16k * 32 -> 512k potential > > requests/coroutines ? > > Yes. This is the number of request descriptors that fit in the > virtqueues, and if you add another device with additional virtqueues, > then obviously that increases the number of theoretically possible > parallel requests. > > The limits of what you can actually achieve in practice might be lower > because I/O might complete faster than the time we need to process all > of the queued requests, depending on how many vcpus are trying to > "compete" with how many iothreads. Of course, the practical limits in > five years might be different from today. > > > > > IIUC, coroutine stack size is 1 MB, and so tens of thousands of > > > > coroutines implies 10's of GB of memory just on stacks alone. > > > > > > That's only virtual memory, though. Not sure how much of it is actually > > > used in practice. > > > > True, by default Linux wouldn't care too much about virtual memory, > > Only if 'vm.overcommit_memory' is changed from its default, such > > that Linux applies an overcommit ratio on RAM, then total virtual > > memory would be relevant. > > That's a good point and one that I don't have a good answer for, short > of just replacing the whole QEMU block layer with rsd and switching to > stackless coroutines/futures this way. > > > > > > Eventually vm.max_map_count is reached and memory-related syscalls > > > > > fail. > > > > > > > > On my system max_map_count is 1048576, quite alot higher than > > > > 10's of 1000's. Hitting that would imply ~500,000 coroutines and > > > > ~500 GB of stacks ! > > > > > > Did you change the configuration some time in the past, or is this just > > > a newer default? I get 65530, and that's the same default number I've > > > seen in the bug reports. > > > > It turns out it is a Fedora change, rather than a kernel change: > > > > https://fedoraproject.org/wiki/Changes/IncreaseVmMaxMapCount > > Good to know, thanks. > > > > > > diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c > > > > > index 5fd2dbaf8b..2790959eaf 100644 > > > > > --- a/util/qemu-coroutine.c > > > > > +++ b/util/qemu-coroutine.c > > > > > > > > > +static unsigned int get_global_pool_hard_max_size(void) > > > > > +{ > > > > > +#ifdef __linux__ > > > > > + g_autofree char *contents = NULL; > > > > > + int max_map_count; > > > > > + > > > > > + /* > > > > > + * Linux processes can have up to max_map_count virtual memory > > > > > areas > > > > > + * (VMAs). mmap(2), mprotect(2), etc fail with ENOMEM beyond > > > > > this limit. We > > > > > + * must limit the coroutine pool to a safe size to avoid running > > > > > out of > > > > > + * VMAs. > > > > > + */ > > > > > + if (g_file_get_contents("/proc/sys/vm/max_map_count", &contents, > > > > > NULL, > > > > > + NULL) && > > > > > + qemu_strtoi(contents, NULL, 10, &max_map_count) == 0) { > > > > > + /* > > > > > + * This is a conservative upper bound that avoids exceeding > > > > > + * max_map_count. Leave half for non-coroutine users like > > > > > library > > > > > + * dependencies, vhost-user, etc. Each coroutine takes up 2 > > > > > VMAs so > > > > > + * halve the amount again. > > > > > + */ > > > > > + return max_map_count / 4; > > > > > > > > That's 256,000 coroutines, which still sounds incredibly large > > > > to me. > > > > > > The whole purpose of the limitation is that you won't ever get -ENOMEM > > > back, which will likely crash your VM. Even if this hard limit is high, > > > that doesn't mean that it's fully used. Your setting of 1048576 probably > > > means that you would never have hit the crash anyway. > > > > > > Even the benchmarks that used to hit the problem don't even get close to > > > this hard limit any more because the actual number of coroutines stays > > > much smaller after applying this patch. > > > > I'm more thinking about what's the worst case behaviour that a > > malicious guest can inflict on QEMU, and cause unexpectedly high > > memory usage in the host. > > > > ENOMEM is bad for a friendy VM, but there's also the risk to the host > > from a unfriendly VM exploiting the high limits > > But from a QEMU perspective, what is the difference between a friendly > high-performance VM that exhausts the available bandwidth to do its job > as good and fast as possible, and a malicious VM that does that same > just to waste host resources? I don't think QEMU can decide this, they > look the same. > > If you want a VM not to send 16k requests in parallel, you can configure > its disk to expose less queues or a smaller queue size. The values I > mentioned above are only defaults that allow friendly VMs to perform > well out of the box, nothing prevents you from changing them to restrict > the amount of resources a VM can use.
Reducing queues is a no-win scenario, as it limits the performance of a single disk when used in isolation, in order to cap the worst case when all disks are used concurrently :-( It would be nice to allow a single disk to burst to a high level, and only limit coroutines if many disks are all trying to concurrently burst to a high level. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|