On Tue, Apr 19, 2011 at 11:31:06PM +0800, Vivek Goyal wrote: > On Tue, Apr 19, 2011 at 11:22:40PM +0800, Wu Fengguang wrote: > > On Tue, Apr 19, 2011 at 11:11:11PM +0800, Vivek Goyal wrote: > > > On Tue, Apr 19, 2011 at 04:48:32PM +0200, Jan Kara wrote: > > > > On Tue 19-04-11 10:34:23, Vivek Goyal wrote: > > > > > On Tue, Apr 19, 2011 at 10:17:17PM +0800, Wu Fengguang wrote: > > > > > > [snip] > > > > > > > > > > For throttling case, apart from metadata, I found that with simple > > > > > > > > > > throttling of data I ran into issues with journalling with ext4 mounuted > > > > > > > > > > in ordered mode. So it was suggested that WRITE IO throttling should > > > > > > > > > > not be done at device level instead try to do it in higher layers, > > > > > > > > > > possibly balance_dirty_pages() and throttle process early. > > > > > > > > > > > > > > > > > > The problem with doing it at the page cache entry level is that > > > > > > > > > cache hits then get throttled. It's not really a an IO controller at > > > > > > > > > that point, and the impact on application performance could be huge > > > > > > > > > (i.e. MB/s instead of GB/s). > > > > > > > > > > > > > > > > Agreed that throttling cache hits is not a good idea. Can we determine > > > > > > > > if page being asked for is in cache or not and charge for IO accordingly. > > > > > > > > > > > > > > You'd need hooks in find_or_create_page(), though you have no > > > > > > > context of whether a read or a write is in progress at that point. > > > > > > > > > > > > I'm confused. Where is the throttling at cache hits? > > > > > > > > > > > > The balance_dirty_pages() throttling kicks in at write() syscall and > > > > > > page fault time. For example, generic_perform_write(), do_wp_page() > > > > > > and __do_fault() will explicitly call > > > > > > balance_dirty_pages_ratelimited() to do the write throttling. > > > > > > > > > > This comment was in the context of what if we move block IO controller read > > > > > throttling also in higher layers. Then we don't want to throttle reads > > > > > which are already in cache. > > > > > > > > > > Currently throttling hook is in generic_make_request() and it kicks in > > > > > only if data is not present in page cache and actual disk IO is initiated. > > > > You can always throttle in readpage(). It's not much higher than > > > > generic_make_request() but basically as high as it can get I suspect > > > > (otherwise you'd have to deal with lots of different code paths like page > > > > faults, splice, read, ...). > > > > > > Yep, I was thinking that what do I gain by moving READ throttling up. > > > The only thing generic_make_request() does not catch is network file > > > systems. I think for that I can introduce another hook say in NFS and > > > I might be all set. > > > > Basically all data reads go through the readahead layer, and the > > __do_page_cache_readahead() function. > > > > Just one more option for your tradeoffs :) > > But this does not cover direct IO? Yes, sorry! > But I guess if I split the hook into two parts (one in direct IO path > and one in __do_page_cache_readahead()), then filesystems don't have > to mark meta data READS. I will look into it. Right, and the hooks should be trivial to add. The readahead code is typically invoked in three ways: - sync readahead, on page cache miss, => page_cache_sync_readahead() - async readahead, on hitting PG_readahead (tagged on one page per readahead window), => page_cache_async_readahead() - user space readahead, fadvise(WILLNEED), => force_page_cache_readahead() ext3/4 also call into readahead on readdir(). The readahead window size is typically 128K, but much larger for software raid, btrfs and NFS, typically multiple MB and even more. Thanks, Fengguang -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
