On Wed, Jan 11, 2017 at 01:39:14PM -0800, Song Liu wrote: > Chunk aligned read significantly reduces CPU usage of raid456. > However, it is not safe to fully bypass the write back cache. > This patch enables chunk aligned read with write back cache. > > For chunk aligned read, we track stripes in write back cache at > a bigger granularity, "big_stripe". Each chunk may contain more > than one stripe (for example, a 256kB chunk contains 64 4kB-page, > so this chunk contain 64 stripes). For chunk_aligned_read, these > stripes are grouped into one big_stripe, so we only need one lookup > for the whole chunk. > > For each big_stripe, struct big_stripe_info tracks how many stripes > of this big_stripe are in the write back cache. We count how many > stripes of this big_stripe are in the write back cache. These > counters are tracked in a radix tree (big_stripe_tree). > r5c_tree_index() is used to calculate keys for the radix tree. > > chunk_aligned_read() calls r5c_big_stripe_cached() to look up > big_stripe of each chunk in the tree. If this big_stripe is in the > tree, chunk_aligned_read() aborts. This look up is protected by > rcu_read_lock(). > > It is necessary to remember whether a stripe is counted in > big_stripe_tree. Instead of adding new flag, we reuses existing flags: > STRIPE_R5C_PARTIAL_STRIPE and STRIPE_R5C_FULL_STRIPE. If either of these > two flags are set, the stripe is counted in big_stripe_tree. This > requires moving set_bit(STRIPE_R5C_PARTIAL_STRIPE) to > r5c_try_caching_write(); and moving clear_bit of > STRIPE_R5C_PARTIAL_STRIPE and STRIPE_R5C_FULL_STRIPE to > r5c_finish_stripe_write_out(). > > Signed-off-by: Song Liu <songliubraving@xxxxxx> applied, thanks! -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
