Re: A lockless pagecache for Linux

[Wu Fengguang <wfg@mail.ustc.edu.cn>]

On Tue, Mar 14, 2006 at 03:14:38PM +1100, Nick Piggin wrote:
> Christoph Lameter wrote:
> >What you are proposing is to allow lockless read operations right? No 
> >lockless write? The concurrency issue that we currently have is multiple 
> >processes faulting in pages in different ranges from the same file. I 
> >think this is a rather typical usage scenario. Faulting in a page from a 
> >file for reading requires a write operation on the radix tree. The 
> >approach with a lockless read path does not help us. This proposed scheme 
> >would only help if pages are already faulted in and another process starts
> >using the same pages as an earlier process.
> >
> 
> Yep, lockless reads only to start with. I think you'll see some benefit
> because the read(2) and ->nopage paths also take read-side locks, so your
> write side will no longer have to contend with them.
> 
> It won't be a huge improvement in scalability though, maybe just a constant
> factor.
> 
> >Would it not be better to handle the radix tree in the same way as a page 
> >table? Have a lock at the lowest layer so that different sections of the 
> >radix tree can be locked by different processes? That would enable 
> >concurrent writes.
> >
> 
> Yeah this is the next step. Note that it is not the first step because I
> actually want to _speed up_ single threaded lookup paths, rather than
> slowing them down, otherwise it will never get accepted.
> 
> It also might add quite a large amount of complexity to the radix tree, so
> it may no longer be suitable for a generic data structure anymore (depends
> how it is implemented). But the write side should be easier than the
> read-side so I don't think there is too much to worry about. I already have
> some bits and pieces to make it fine-grained.

Maybe we can try another way to reduce the concurrent radix tree
writers problem: coordinating and serializing writers at high level.

Since kswapd is the major radix tree deleter, and readahead is the
major radix tree inserter, putting parts of them together in a loop
might reduce the contention noticeably.

The following pseudo-code shows the basic idea:
(Integrating kprefetchd is also possible. Just for simplicity...:)

PER_NODE(ra_queue);

kswapd()
{
        loop {
                loop {
                        free enough pages for top(ra_queue)
                }
                submit(pop(ra_queue));
                wait();
        }
}

readahead()
{
        assemble one ra_req

        if (ra_req immediately needed)
                submit(ra_req);
        else {
                push(ra_queue, ra_req);
                wakeup_kswapd();
        }
}

This scheme might reduce
        - direct page reclaim pressure
        - radix tree write lock contention
        - lru lock contention

Regards,
Wu
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