reduce lock contention in do_pagecache_readahead
Subsystems (2)
PATCH 1/15 reduce lock contention in do_pagecache_readahead
Anton Blanchard has a workload (the SDET benchmark) which is showing some moderate lock contention in do_pagecache_readahead().
Seems that SDET has many threads performing seeky reads against a cached file. The average number of pagecache probes in a single do_pagecache_readahead() is six, which seems reasonable.
The patch (from Anton) flips the locking around to optimise for the fast case (page was present). So the kernel takes the lock less often, and does more work once it has been acquired.
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mm/readahead.c renamed from mm/readahead.c~anton-readahead-locking +4 -2
@@ -117,25 +117,27 @@ void do_page_cache_readahead(struct file/* * Preallocate as many pages as we will need. */ +read_lock (&mapping->page_lock);for (page_idx =0 ; page_idx < nr_to_read; page_idx++) {unsigned long page_offset = offset + page_idx;if (page_offset > end_index)break ; -read_lock (&mapping->page_lock); page =radix_tree_lookup (&mapping->page_tree, page_offset); -read_unlock (&mapping->page_lock);if (page)continue ; +read_unlock (&mapping->page_lock); page =page_cache_alloc (mapping); +read_lock (&mapping->page_lock);if (!page)break ; page->index = page_offset;list_add (&page->list, &page_pool); nr_to_really_read++; } +read_unlock (&mapping->page_lock);/* * Now start the IO. We ignore I/O errors - if the page is not
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