The swapping subsusytem has few components
1. Activation of page selection (periodic/explicit)
2. Page reclaim (policy)
3. Reverse Mapping
4. Swap cache
5. Writeback mechanism
a little distantly related: fault on a swapped page (notes)
code pages are file pages and kept track of by file address space object. difference between a code page and a data page is not by the backing file. but it is whether that page is editable in memory or not. if not editable, then it is a code page (most probably), if it is editable then it is a data page. now editable data page is synchronized with backing block device during swapping. but non-editable code pages are just discarded during swapping.
process heap, stack are anonymous pages because they do not have any backing block device store. memory area can be mapped anonymously using mmap too. these anon pages are swapped out to swap area.
private mapping is an exception because in this case data is on the backing file but private mapping decouples the data from the file. so private mapped pages are swapped out to swap area.
IPC shared pages are also swapped to swap area.
PFRA is all about identifying which data is more important than others. less important data is swapped out to swap area if required. From the viewpoint of a process, OS need to find out the working set of a process. there might be read working set (PCM-based) and write working set (DRAM-based).
PFRA needs to be 1) fair and 2) scalable
there is an idea of swap token, which makes a process immune against swapping because OS tries not to swap out that process' pages, thereby making that process run faster.
there can be multiple swap areas with different priorities depending on their relative speed. to keep track of slots in swap area, kernel uses bitmap. all swap files/areas are called swap files in kernel terminology.
each zone has an active and an inactive list of pages.
shrink_zone() is just one function which does a periodical move of pages from active to inactive lists. A page is first regarded as inactive, but it must earn some importance to be considered active.
1. Activation of page selection (periodic/explicit)
2. Page reclaim (policy)
3. Reverse Mapping
4. Swap cache
5. Writeback mechanism
a little distantly related: fault on a swapped page (notes)
code pages are file pages and kept track of by file address space object. difference between a code page and a data page is not by the backing file. but it is whether that page is editable in memory or not. if not editable, then it is a code page (most probably), if it is editable then it is a data page. now editable data page is synchronized with backing block device during swapping. but non-editable code pages are just discarded during swapping.
process heap, stack are anonymous pages because they do not have any backing block device store. memory area can be mapped anonymously using mmap too. these anon pages are swapped out to swap area.
private mapping is an exception because in this case data is on the backing file but private mapping decouples the data from the file. so private mapped pages are swapped out to swap area.
IPC shared pages are also swapped to swap area.
PFRA is all about identifying which data is more important than others. less important data is swapped out to swap area if required. From the viewpoint of a process, OS need to find out the working set of a process. there might be read working set (PCM-based) and write working set (DRAM-based).
PFRA needs to be 1) fair and 2) scalable
there is an idea of swap token, which makes a process immune against swapping because OS tries not to swap out that process' pages, thereby making that process run faster.
there can be multiple swap areas with different priorities depending on their relative speed. to keep track of slots in swap area, kernel uses bitmap. all swap files/areas are called swap files in kernel terminology.
each zone has an active and an inactive list of pages.
shrink_zone() is just one function which does a periodical move of pages from active to inactive lists. A page is first regarded as inactive, but it must earn some importance to be considered active.
No comments:
Post a Comment