Messages in this thread | | | From | "William J. Earl" <> | Date | Thu, 9 Dec 1999 16:30:48 -0800 (PST) | Subject | Re: Getting big areas of memory, in 2.3.x? |
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Alan Cox writes: ... > > for higher-bandwidth targets, such as a graphics controller or a > > HDTV camera. > > I don't know of any capture cards that don't do scatter gather. Most of them > do scatter gather with skipping and byte alignment so you can DMA around > other windows. > > This is the main point. There are so so few devices that actually _have_ to > have lots of linear memory it is questionable that it is worth paying the > price to allow modules to allocate that way
If the only issue were devices which cannot do scatter-gather, I would certainly agree. However, except for the SGI O2 (which only cares about 64 KB pages in hardware, anyway), all of the SGI hardware has been happy to do scatter-gather. What we found with (high resolution) digital media and other applications which do a lot of large DMAs was that the overhead of doing the equivalent of map_kiobuf()/unmap_kiobuf() for large buffers composed of many small pages was substantial, compared to doing it for large buffers composed of large pages. Admittedly, the IRIX equivalent is less efficient than map_kiobuf(), but map_kiobuf() does still have to touch a lot of cache lines when visiting all of the small pages in a large buffer.
Then too, there is the matter of TLB misses for applications which visit a lot of data, especially on processors with reasonably large caches. With 4 KB pages and 64 TLB entries, the TLB cannot map all of a cache larger than 256 KB. If the cache is, say, 2 MB and the application cycles through many of the pages in the cache in a loop, you can wind up with a TLB miss for almost every load (other than those from the stack). With 1 MB pages, there are almost no TLB misses.
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