>> Why doesn't the kernel use two memory blocks whose size is 2048KB(i.e.*oder 9 *) >> instead of one block *order 10 *(you see, there are still three free blocks and >> 2048KB*2=4096KB equivalent to the memory size of order 10)? > >Most parts of the kernel, when asking for very high-order allocations, *will* >have a fallback strategy to use smaller chunks. So, for instance, if a device >need a 1M buffer and supports scatter-gather operations, if 1M of contiguous >memory isn't available, the kernel can ask for 4 256K chunks and have the I/O >directed into the 4 areas. *However, if the memory *has* to be contiguous (for* >*example, no scatter/gather available, or it's for an array data structure),* >then it can't do that.
Thank you for the clarification. I understand it on a deeper level with your help. How can I know whether scatter/gather is available or not? In another word, when it's available and when it's not? I do not intend to ask the behavior of gadget driver. I just wonder how I can confirm it in general. Thank you for your attention to this matter. Look forward to hearing from you. Best regards. Valdis Klētnieks <[email protected]> 于2020年6月19日周五 下午3:14写道： > On Fri, 19 Jun 2020 14:56:20 +0800, 孙世龙 sunshilong said: > > > Why doesn't the kernel use two memory blocks whose size is > 2048KB(i.e.*oder 9 *) > > instead of one block *order 10 *(you see, there are still three free > blocks and > > 2048KB*2=4096KB equivalent to the memory size of order 10)? > > Most parts of the kernel, when asking for very high-order allocations, > *will* > have a fallback strategy to use smaller chunks. So, for instance, if a > device > need a 1M buffer and supports scatter-gather operations, if 1M of > contiguous > memory isn't available, the kernel can ask for 4 256K chunks and have the > I/O > directed into the 4 areas. However, if the memory *has* to be contiguous > (for > example, no scatter/gather available, or it's for an array data structure), > then it can't do that. > > And in fact, that fallback could very well have happened in this case - I > didn't bother chasing back to see if the gadget driver does recovery by > allocating multiple smaller chunks. > > (That's a good "exercise for the student"... :) >
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