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Editorial Team
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Asked: 2026-06-11T02:56:03+00:00

I would like to use DMA to accelarate network I/O (intensive disk reads and

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I would like to use DMA to accelarate network I/O (intensive disk reads and output via Internet). I wonder if I have a multithreaded application where each thread issues DMA transfers how does the disk I/O and DMA transfers are scheduled by the OS?

As far as I understand, the kernel loads the required portion of data to RAM from disk without user program and issues DMA transfer. Hence, a disk scheduler will not help a lot (only a single kernel process always accesses the disk). Also may I hope that there are any in-kernel facilities to schedule DMA transfers when for the data that was loaded into memory?

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1 Answer

  1. Editorial Team

    The network driver will already be using DMA to accelerate to transfers. When you issue a write the kernel will allocate a contiguous block of physical memory and copy the data from your userspace buffer into this memory. During this phase the kernel will attach all the necessary Ethernet and TCP/IP headers.

    The kernel will then issue a DMA request to the network card, asking it to take data from that physical memory location and load it into its internal buffers. At this point your write system call will return. When the network card is complete (and the data is on its way out of the adapter) the network card will signal completion to the kernel.

    In Linux network drivers are normally single threaded (there are some exceptions to this but it gets complicated), so if you try to write some data and the driver is already active it will still be copied into kernel space but the DMA request will not be performed until the network driver is free again (it’ll be triggered when the kernel is next notified that a DMA is complete).

    The morale of the story is that this already works and is rather fast, there’s nothing you need to do to accelerate and application using DMA, it’s already been taken care of. The only piece you could speed up would be the copy in the kernel space buffer, but as this is so much quicker than the actual network transfer (and can be done simultaneously) it doesn’t make any difference to throughput, only latency.

    N.B. The above is a gross simplification in places, if you want more detail about a specific part edit your question and I’ll do what I can.

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