Messages in this thread | | | Date | Tue, 14 Mar 2006 20:59:15 +0800 | From | Wu Fengguang <> | Subject | Re: A lockless pagecache for Linux |
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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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