Messages in this thread | | | Date | Mon, 6 Aug 2007 13:27:47 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 02/10] mm: system wide ALLOC_NO_WATERMARK |
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On Mon, 6 Aug 2007 13:19:26 -0700 (PDT) Christoph Lameter <clameter@sgi.com> wrote:
> On Mon, 6 Aug 2007, Matt Mackall wrote: > > > > > Because a block device may have deadlocked here, leaving the system > > > > unable to clean dirty memory, or unable to load executables over the > > > > network for example. > > > > > > So this is a locking problem that has not been taken care of? > > > > No. > > > > It's very simple: > > > > 1) memory becomes full > > We do have limits to avoid memory getting too full. > > > 2) we try to free memory by paging or swapping > > 3) I/O requires a memory allocation which fails because memory is full > > 4) box dies because it's unable to dig itself out of OOM > > > > Most I/O paths can deal with this by having a mempool for their I/O > > needs. For network I/O, this turns out to be prohibitively hard due to > > the complexity of the stack. > > The common solution is to have a reserve (min_free_kbytes). The problem > with the network stack seems to be that the amount of reserve needed > cannot be predicted accurately. > > The solution may be as simple as configuring the reserves right and > avoid the unbounded memory allocations. That is possible if one > would make sure that the network layer triggers reclaim once in a > while.
Such a simple fix would be attractive. Some of the net drivers now have remarkably large rx and tx queues. One wonders if this is playing a part in the problem and whether reducing the queue sizes would help.
I guess we'd need to reduce the queue size on all NICs in the machine though, which might be somewhat of a performance problem.
I don't think we've seen a lot of justification for those large queues. I'm suspecting it's a few percent in carefully-chosen workloads (of the microbenchmark kind?)
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