Re: I/O Request [Elevator; Clustering; Scatter-Gather]

[Luben Tuikov <luben@splentec.com>]

Bryan Henderson wrote:
> 
> 
> 
>>>Scatter-gather may cluster sg entries that are
>>>
>>>>contiguous in memory.
> 
> Please explain this function.
>

But, he did explain it.

Basically if you have two or more sg entries (vaguely
a buffer address and buffer length) *and* they are contiguous
in memory (tricky), i.e. (100,100), (200,100), (300,100),
then a smart choice would be to make just a single sg
entry (100,300). (This was just an example for illustratory
puposes.)

For the other concepts, the book
"Operating System Concepts", Silberschatz & Galvin,
Addison-Wesley, 1998, ISBN 0-201-59113-8, commonly
referred to as ``the dinosaur book'' (because of the
dinosaurs depicted on the cover) is a good start.


> The only "scatter gather" I've heard of is the concept of a single I/O
> having a discontiguous buffer.  If it's a write, the system gathers bytes
> from multiple separate buffers into a single write.  If it's a read, the
> system scatters the bytes from a single read into multiple separate
> buffers.
> 
> The readv() and writev() system calls are examples of something that
> implements a scatter/gather function.  (But not the scatter/gather function
> we're talking about here).
> 
> When you cluster disk I/O by combining separate requests that happen to be
> to adjacent disk locations, you must have a scatter/gather function
> somewhere (e.g. the disk controller or DMA controller) to make it work,
> because those requests probably don't have contiguous buffers.
> 
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-- 
Luben