Re: [RFC] first try for swap prefetch

[Thomas Schlichter <schlicht@uni-mannheim.de>]

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On April 11, Andrew Morton wrote:
> Thomas Schlichter <schlicht@uni-mannheim.de> wrote:
> > Hi,
> >
> > as mentioned a few days ago I was going to try to implement a swap
> > prefetch to better utilize the free memory. Now here is my first try.
>
> That's surprisingly cute.  Does it actually do anything noticeable?

Well, it fills free pagecache memory with swapped pages... ;-)

But at the moment I can not 'feel' any real improvement... :-(
I think the problem is that R/O pages are not written to swap space and so not 
prefetched with my patch. But I will look after it...

> +	swapped_entry = kmalloc(sizeof(*swapped_entry), GFP_ATOMIC);
>
> These guys will need a slab cache (not SLAB_HW_CACHE_ALIGNED) to save
> space.

OK, I'll do it.

> +	swapped_entry = radix_tree_lookup(&swapped_root.tree, entry.val);
> +	if(swapped_entry) {
> +		list_del(&swapped_entry->list);
> +		radix_tree_delete(&swapped_root.tree, entry.val);
>
> you can just do
>
> 	if (radix_tree_delete(...) != -ENOENT)
> 		list_del(...)
>
> +		read_swap_cache_async(entry);

Sorry, but I think I can not. The list_del() needs the value returned by 
radix_tree_lookup(), so I can not kick it... By the way, the only reason for 
the radix tree is to make this list_del() not O(n) for searching the list... 
Do you know how expensive the radix_tree_lookup() is? O(1) or O(log(n))?? For 
my shame I do not really know that data structure... :-(

> What you want here is a way of telling if the disk(s) which back the swap
> are idle.  We used to have that, but Hugh deleted it.  It can be put back,
> but it's probably better to put a `last_read_request_time' and
> `last_write_request_time' into struct backing_dev_info.  If nobody has used
> the disk in the past N milliseconds, then start the speculative swapin.

That's good. I was looking for anything like that but didn't find anything 
fitting in the current sources...

> It might make sense to poke the speculative swapin code in the page-freeing
> path too.

I wanted to do this but don't know which function is the correct one for this. 
But I will search harder... or can you give me a hint?

> And to put the speculatively-swapped-in pages at the tail of the inactive
> list (perhaps).

This may be a good idea...

> But first-up, some demonstrated goodness is needed...

Yup, but currently it improves nothing very much, as stated above, I think 
first I should implement the R/O pages thing and investigete which part of 
the kernel works against my code and frees some pages after I just filled 
them...

Thank you for helping me with your comments!

Best regards
   Thomas

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