Can swap-in be improved? [LWN.net]

Can swap-in be improved?

Posted May 31, 2007 17:04 UTC (Thu) by anton (guest, #25547)
In reply to: Can swap-in be improved? by giraffedata
Parent article: On-demand readahead

A lot of it is reading in memory mapped program files. Speeding that up isn't as easy.

I think it's not very hard. You just need some history-based prediction mechanism. A student of mine has done his Master's thesis (in German) on speeding up the startup time of programs by recording the blocks accessed when starting a program, and then prefetching them (Windows XP does something like that for the OS startup). Similarly, one could record the pages or blocks accessed soon after a major page fault, and then prefetch all of them the next time the page faults.

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Can swap-in be improved?

Posted Jun 2, 2007 2:30 UTC (Sat) by giraffedata (subscriber, #1954) [Link]

That's actually different, and a little harder, than what we're talking about here. We're talking about resuming a process that has been idle for a while. So all the history you need is the list of pages that were resident the last time the process ran.

That by itself can't get you to the level of speed that we discussed with anonymous pages, where all those pages were contiguous on disk.

I know I've seen somewhere a system where you don't ordinarily start a program, but rather reload and resume an entire process. In that case, the two prefetching problems are probably the same.

Can swap-in be improved?

Posted Jun 7, 2007 13:29 UTC (Thu) by anton (guest, #25547) [Link]

Yes, fetching all the pages that were resident is certainly an approach for the problem that would work.

Using a history-based prefetcher might have the following advantages, though:

It could fetch fewer pages (e.g., not pages loaded on startup, but no longer needed, but that were still resident when the process was swapped out), which would be faster and less disruptive for other processes.
The prefetcher would also be useful in other situations, like on startup, or when a process enters a new phase (and therefore needs many new pages from executable or data files).

That being said, a well-balanced history-based prefetcher is certainly a much larger project than loading the once-resident pages.