Re: [dpdk-dev] [PATCH 3/3] lib/hash: adjust tbl_chng_cnt position

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> >
> >tbl_chng_cnt is one of the first elements of the structure used in the
> >lookup. Move it to the beginning of the cache line to gain performance.
> >
> >Fixes: e605a1d36 ("hash: add lock-free r/w concurrency")
> >Cc: stable@dpdk.org
> >
> >Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> >Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> >Tested-by: Ruifeng Wang <ruifeng.wang@arm.com>
> >---
> > lib/librte_hash/rte_cuckoo_hash.h | 6 +++---
> > 1 file changed, 3 insertions(+), 3 deletions(-)
> >
> >diff --git a/lib/librte_hash/rte_cuckoo_hash.h
> >b/lib/librte_hash/rte_cuckoo_hash.h
> >index fb19bb27d..af6451b5c 100644
> >--- a/lib/librte_hash/rte_cuckoo_hash.h
> >+++ b/lib/librte_hash/rte_cuckoo_hash.h
> >@@ -170,7 +170,9 @@ struct rte_hash {
> >
> > 	/* Fields used in lookup */
> >
> >-	uint32_t key_len __rte_cache_aligned;
> >+	uint32_t *tbl_chng_cnt __rte_cache_aligned;
> >+	/**< Indicates if the hash table changed from last read. */
> >+	uint32_t key_len;
> > 	/**< Length of hash key. */
> > 	uint8_t hw_trans_mem_support;
> > 	/**< If hardware transactional memory is used. */ @@ -218,8 +220,6
> @@
> >struct rte_hash {
> > 	 * is piggy-backed to freeing of the key index.
> > 	 */
> > 	uint32_t *ext_bkt_to_free;
> >-	uint32_t *tbl_chng_cnt;
> >-	/**< Indicates if the hash table changed from last read. */
> > } __rte_cache_aligned;
> >
> > struct queue_node {
> >--
> >2.17.1
> 
> [Wang, Yipeng]
> I am not sure about this change. By moving counter to front, I think you
> seems push key_store out of the cache line. And key_store Is also used in
> lookup (and more commonly).
> My tests also show perf drop in many cases.
I ran hash_readwrite_lf tests and L3 fwd application. Both of them showed improvements for both lock-free and using locks for Arm platforms (L3 fwd was not run on x86). Which tests are resulting in performance drops for you?

But, I do agree that this work is not complete. We can drop this patch and take this up separately.