Messages in this thread Patch in this message |  | | | Date | Thu, 03 Mar 2005 13:08:42 +0300 | | From | Oleg Nesterov <> | | Subject | Re: [PATCH 1/2] readahead: simplify ra->size testing |
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Andrew Morton wrote: > > So... the big "how it all works" comment needs an update..
Same patch, comment updated.
Currently page_cache_readahead() treats ra->size == 0 (first read) and ra->size == -1 (ra_off was called) separately, but does exactly the same in both cases.
With this patch we may assume that the reading starts in 'ra_off()' state, so we don't need to consider the first read as a special case.
file_ra_state_init() sets ra->prev_page = -1; ra->size = 0;
When the page_cache_readahead() is called for the first time it sets ra->size to nonzero value either via get_init_ra_size() or ra_off(). So ra->size == 0 implies that ra->prev_page == -1. I am ignoring the case when readahead is disabled via ra->ra_pages == 0.
page_cache_readahead detects sub-page sized reads: if (offset == ra->prev_page && req_size == 1 && ra->size != 0) But if offset == ra->prev_page, then ra->size == 0 can happen only if offset == -1, so there is no need to check ra->size here. If application starts reading 16Tb file from the last page then readahead can't help.
First offset==0 read or first sequential detection: if ((ra->size == 0 && offset == 0) || (ra->size == -1 && sequential) could be changed to: if ((ra->size == 0 && sequential) || (ra->size == -1 && sequential) which means: if (sequential && (ra->size == 0 || ra->size == -1))
Random case detection: if (!sequential || (ra->size == 0)) But if sequential == 1, then ra->size can't be 0, this case is already handled before.
Now we have:
if (offset == ra->prev_page && req_size == 1) /* sub-page reads */ if (sequential && (ra->size == 0 || ra->size == -1)) /* first offset==0 read or first sequential */ if (!sequential) /* random case */ Now ->size is checked only in one place, so ra_off() can set ra->size = 0, and we can just test ->size against 0.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
--- 2.6.11/mm/readahead.c~ 2005-02-04 21:33:40.000000000 +0300 +++ 2.6.11/mm/readahead.c 2005-02-04 21:33:57.000000000 +0300 @@ -55,7 +55,7 @@ static inline void ra_off(struct file_ra { ra->start = 0; ra->flags = 0; - ra->size = -1; + ra->size = 0; ra->ahead_start = 0; ra->ahead_size = 0; return; @@ -452,7 +452,7 @@ page_cache_readahead(struct address_spac * perturbing the readahead window expansion logic. * If size is zero, there is no read ahead window so we need one */ - if (offset == ra->prev_page && req_size == 1 && ra->size != 0) + if (offset == ra->prev_page && req_size == 1) goto out; ra->prev_page = offset; @@ -471,9 +471,7 @@ page_cache_readahead(struct address_spac * at start of file, and grow the window fast. Or detect first * sequential access */ - if ((ra->size == 0 && offset == 0) /* first io and start of file */ - || (ra->size == -1 && sequential)) { - /* First sequential */ + if (sequential && ra->size == 0) { ra->size = get_init_ra_size(newsize, max); ra->start = offset; if (!blockable_page_cache_readahead(mapping, filp, offset, @@ -499,7 +497,7 @@ page_cache_readahead(struct address_spac * partial page reads and first access were handled above, * so this must be the next page otherwise it is random */ - if (!sequential || (ra->size == 0)) { + if (!sequential) { ra_off(ra); blockable_page_cache_readahead(mapping, filp, offset, newsize, ra, 1); - 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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