Agner`s CPU blog

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Paging
Author: Agner Date: 2016-09-10 00:18
Kurt Baumgardner wrote:
my worry about the decision to drop paging from the memory model,
The current systems have a very complicated memory paging systems with huge two-level page tables which are partially cached in the on-chip TLB. This system is complicated and using a lot of resources.

My goal with ForwardCom is to see what can be gained by a complete redesign. The paging system seemed an obvious candidate for simplification so I got the idea of replacing the huge number of fixed-size pages with a small number of variable-size pages. It was just an idea that I wanted to discuss, and as you can see it has already generated a lot of discussion.

This memory map system will only work if we can avoid heavy memory fragmentation, but if it works it will be a great advantage. The memory map will be so small that it can be contained entirely on the chip. You can have a separate memory map for each thread and you can easily switch memory map on system calls and task switches.

An application can still allocate memory dynamically, but preferably in a way that doesn't cause too much fragmentation. If an application allocates a huge amount of memory, not knowing how much it will actually need, then the OS may just allocate virtual memory in the first place. When physical memory is needed, it will be allocated in blocks of exponentially growing sizes (each new block will be bigger than the previous one) in order to minimize memory fragmentation. As a last resort, the system may move data around in memory if a contiguous virtual memory space has become mapped to a too fragmented physical memory. Moving data in memory may be fast because the data path to cache (and to main memory?) can be as wide as the maximum vector length.

The advantage of having variable size memory map entries is that you will need much fewer entries. The disadvantage is that you need a lot of comparators to check all entries in one or a few clock cycles. The actual number of entries we will have depends on the hardware implementation and this is subject to experimentation. A well behaved application that causes little or no memory fragmentation will need very few memory map entries. So a programmer who makes a performance-critical application should exercise a little discipline to avoid excessive memory fragmentation.

 
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