readahead: make context readahead more conservative

Try to prevent negatively impact moderately dense random reads on SSD.

Transaction-Per-Second numbers provided by Taobao:

		QPS	case
		-------------------------------------------------------
		7536	disable context readahead totally

w/ patch: 7129 slower size rampup and start RA on the 3rd read
6717 slower size rampup
w/o patch: 5581 unmodified context readahead

Before, readahead will be started whenever reading page N+1 when it happen to read N recently. After patch, we'll only start readahead when *three* random reads happen to access pages N, N+1, N+2. The probability of this happening is extremely low for pure random reads, unless they are very dense, which actually deserves some readahead.

Also start with a smaller readahead window. The impact to interleaved sequential reads should be small, because for a long run stream, the the small readahead window rampup phase is negletable.

The context readahead actually benefits clustered random reads on HDD whose seek cost is pretty high. However as SSD is increasingly used for random read workloads it's better for the context readahead to concentrate on interleaved sequential reads.

---
 mm/readahead.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

Post for review first, will include this in the next readahead series.

@@ -594,10 +594,10 @@ static int try_context_readahead(struct 
 	size = count_history_pages(mapping, ra, offset, max);
 
 	/*
-	 * no history pages:
+	 * not enough history pages:
 	 * it could be a random read
 	 */
-	if (!size)
+	if (size <= req_size)
 		return 0;
 
 	/*

@@ -609,8 +609,8 @@ static int try_context_readahead(struct 
 
 	ra->pattern = RA_PATTERN_CONTEXT;
 	ra->start = offset;
-	ra->size = get_init_ra_size(size + req_size, max);
-	ra->async_size = ra->size;
+	ra->size = min(size + req_size, max);
+	ra->async_size = 1;
 
 	return 1;
 }