diff options
Diffstat (limited to 'mm/readahead.c')
| -rw-r--r-- | mm/readahead.c | 174 |
1 files changed, 174 insertions, 0 deletions
diff --git a/mm/readahead.c b/mm/readahead.c index 072ce8f8357..c094e4f5a25 100644 --- a/mm/readahead.c +++ b/mm/readahead.c @@ -611,3 +611,177 @@ unsigned long ra_submit(struct file_ra_state *ra, return actual; } EXPORT_SYMBOL_GPL(ra_submit); + +/* + * Get the previous window size, ramp it up, and + * return it as the new window size. + */ +static unsigned long get_next_ra_size2(struct file_ra_state *ra, + unsigned long max) +{ + unsigned long cur = ra->readahead_index - ra->ra_index; + unsigned long newsize; + + if (cur < max / 16) + newsize = cur * 4; + else + newsize = cur * 2; + + return min(newsize, max); +} + +/* + * On-demand readahead design. + * + * The fields in struct file_ra_state represent the most-recently-executed + * readahead attempt: + * + * |-------- last readahead window -------->| + * |-- application walking here -->| + * ======#============|==================#=====================| + * ^la_index ^ra_index ^lookahead_index ^readahead_index + * + * [ra_index, readahead_index) represents the last readahead window. + * + * [la_index, lookahead_index] is where the application would be walking(in + * the common case of cache-cold sequential reads): the last window was + * established when the application was at la_index, and the next window will + * be bring in when the application reaches lookahead_index. + * + * To overlap application thinking time and disk I/O time, we do + * `readahead pipelining': Do not wait until the application consumed all + * readahead pages and stalled on the missing page at readahead_index; + * Instead, submit an asynchronous readahead I/O as early as the application + * reads on the page at lookahead_index. Normally lookahead_index will be + * equal to ra_index, for maximum pipelining. + * + * In interleaved sequential reads, concurrent streams on the same fd can + * be invalidating each other's readahead state. So we flag the new readahead + * page at lookahead_index with PG_readahead, and use it as readahead + * indicator. The flag won't be set on already cached pages, to avoid the + * readahead-for-nothing fuss, saving pointless page cache lookups. + * + * prev_index tracks the last visited page in the _previous_ read request. + * It should be maintained by the caller, and will be used for detecting + * small random reads. Note that the readahead algorithm checks loosely + * for sequential patterns. Hence interleaved reads might be served as + * sequential ones. + * + * There is a special-case: if the first page which the application tries to + * read happens to be the first page of the file, it is assumed that a linear + * read is about to happen and the window is immediately set to the initial size + * based on I/O request size and the max_readahead. + * + * The code ramps up the readahead size aggressively at first, but slow down as + * it approaches max_readhead. + */ + +/* + * A minimal readahead algorithm for trivial sequential/random reads. + */ +static unsigned long +ondemand_readahead(struct address_space *mapping, + struct file_ra_state *ra, struct file *filp, + struct page *page, pgoff_t offset, + unsigned long req_size) +{ + unsigned long max; /* max readahead pages */ + pgoff_t ra_index; /* readahead index */ + unsigned long ra_size; /* readahead size */ + unsigned long la_size; /* lookahead size */ + int sequential; + + max = ra->ra_pages; + sequential = (offset - ra->prev_index <= 1UL) || (req_size > max); + + /* + * Lookahead/readahead hit, assume sequential access. + * Ramp up sizes, and push forward the readahead window. + */ + if (offset && (offset == ra->lookahead_index || + offset == ra->readahead_index)) { + ra_index = ra->readahead_index; + ra_size = get_next_ra_size2(ra, max); + la_size = ra_size; + goto fill_ra; + } + + /* + * Standalone, small read. + * Read as is, and do not pollute the readahead state. + */ + if (!page && !sequential) { + return __do_page_cache_readahead(mapping, filp, + offset, req_size, 0); + } + + /* + * It may be one of + * - first read on start of file + * - sequential cache miss + * - oversize random read + * Start readahead for it. + */ + ra_index = offset; + ra_size = get_init_ra_size(req_size, max); + la_size = ra_size > req_size ? ra_size - req_size : ra_size; + + /* + * Hit on a lookahead page without valid readahead state. + * E.g. interleaved reads. + * Not knowing its readahead pos/size, bet on the minimal possible one. + */ + if (page) { + ra_index++; + ra_size = min(4 * ra_size, max); + } + +fill_ra: + ra_set_index(ra, offset, ra_index); + ra_set_size(ra, ra_size, la_size); + + return ra_submit(ra, mapping, filp); +} + +/** + * page_cache_readahead_ondemand - generic file readahead + * @mapping: address_space which holds the pagecache and I/O vectors + * @ra: file_ra_state which holds the readahead state + * @filp: passed on to ->readpage() and ->readpages() + * @page: the page at @offset, or NULL if non-present + * @offset: start offset into @mapping, in PAGE_CACHE_SIZE units + * @req_size: hint: total size of the read which the caller is performing in + * PAGE_CACHE_SIZE units + * + * page_cache_readahead_ondemand() is the entry point of readahead logic. + * This function should be called when it is time to perform readahead: + * 1) @page == NULL + * A cache miss happened, time for synchronous readahead. + * 2) @page != NULL && PageReadahead(@page) + * A look-ahead hit occured, time for asynchronous readahead. + */ +unsigned long +page_cache_readahead_ondemand(struct address_space *mapping, + struct file_ra_state *ra, struct file *filp, + struct page *page, pgoff_t offset, + unsigned long req_size) +{ + /* no read-ahead */ + if (!ra->ra_pages) + return 0; + + if (page) { + ClearPageReadahead(page); + + /* + * Defer asynchronous read-ahead on IO congestion. + */ + if (bdi_read_congested(mapping->backing_dev_info)) + return 0; + } + + /* do read-ahead */ + return ondemand_readahead(mapping, ra, filp, page, + offset, req_size); +} +EXPORT_SYMBOL_GPL(page_cache_readahead_ondemand); |