Limitation of buffer allocation

Hi all,

I am new to linux and his internals.
Thus sorry for my popular technical description.

we deal with an streaming application, whereby some streams of data are received via a high speed network interface card and written to the hard disk. Alle data are only written once and later processed. By means of "vmstat 1" we found out, that all available free memory is allocated for buffer cache. Allocation starts, when first data are received and written to the disk. Allocation ends until about 1 MB is left free.
This kind of allocation is a big problem for the driver of the network interface card. It allocates memory dynamically by means of "alloc_skb".

If there is a ad-hoc requierement of a bigger memory block, this function seems to be unable to take away memory from the buffer cache. Thus alloc_skb returns null pointer, because it must be invoked with flag "GFP_ATOMIC" (sudden return if there is no memory available). As a result the driver runs into a faulty deadlocked state.
We work with kernel 2.2.13.
Furthermore we found out that the limitation of memory allocation for the buffer cache solves the problem. If there is about 2 MB memory left free for the driver, it never runs into the problem.
We tested some ways to limit buffer allocation:
1)
A tiny user program allocates permanent memory and frees it immediately.

The user program takes the memory from the buffers. Thus it is available

for the driver.
It is straightforward with unknown side effects, but it works.
2)
We changed the values in /proc/sys/vm/freepages.
The min value is the memory space, reserved for the kernel and thus for the driver. This space can't allocated for buffers. Increasing the values reserves more space for the kernel. It works too, and seems to be a fine solution.
3)
We tried to change the values in /proc/sys/vm/buffermem. (Values are described in /usr/src/linux/Documentation/proc.txt )
But these values dont have any influence on the buffer allocation. Why?

Any comments?

Do you see problems in changing the values in /proc/sys/vm/freepages? Is there any other solution?

Thanks in advance and bye,

Peter.

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Hi,

Yes, that's all working exactly as expected.

GFP_ATOMIC allocations are always expected to fail immediately if there is insufficient memory available. If you want to keep a larger-than-normal amount of free space available for atomica llocations, then increase the free pages count in /proc/sys/vm/freepages.

No, that's precisely what you are supposed to do!

--Stephen
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