>-----Original Message-----
>From: Honnappa Nagarahalli [mailto:[email protected]]
>Sent: Monday, November 5, 2018 10:08 PM
>To: Jerin Jacob <[email protected]>
>Cc: Richardson, Bruce <[email protected]>; De Lara Guarch, Pablo
><[email protected]>; [email protected]; Wang,
>Yipeng1 <[email protected]>; Dharmik Thakkar <[email protected]>;
>Gavin Hu (Arm Technology China)
><[email protected]>; nd <[email protected]>; [email protected]; Yigit, Ferruh
><[email protected]>;
>[email protected]; [email protected]; nd <[email protected]>
>Subject: RE: [dpdk-dev] [PATCH v7 4/5] hash: add lock-free read-write
>concurrency
>> >
>> > 9) Does anyone else facing this problem?
>Any data on x86?
>
[Wang, Yipeng]
I tried Jerin's tests on x86. So by default l3fwd on x86 will use lookup_bulk
and SIMD instruction so there is no obvious throughput
drop on both hit and miss cases (for hit case, there is about 2.5% drop though).
I manually changed l3fwd to do single packet lookup instead of bulk. For hit
case there is no throughput drop.
For miss case, there is 10% throughput drop.
I dig into it, as expected, atomic load indeed translates to regular mov on
x86.
But since the reordering of the instruction, the compiler(gcc 5.4)
cannot unroll the for loop to a switch-case like assembly as before.
So I believe the reason of performance drops on x86 is because compiler cannot
optimize the code as well as previously.
I guess this is totally different reason from why
your performance drop on non-TSO machine. On non-TSO machine, probably the
excessive number of atomic load
causes a lot of overhead.
A quick fix I found useful on x86 is to read all index together. I am no expert
on the use of atomic intinsics, but I assume
By adding a fence should still maintain the correct ordering?
- uint32_t key_idx;
+ uint32_t key_idx[RTE_HASH_BUCKET_ENTRIES];
void *pdata;
struct rte_hash_key *k, *keys = h->key_store;
+ memcpy(key_idx, bkt->key_idx, 4 * RTE_HASH_BUCKET_ENTRIES);
+ __atomic_thread_fence(__ATOMIC_ACQUIRE);
+
for (i = 0; i < RTE_HASH_BUCKET_ENTRIES; i++) {
- key_idx = __atomic_load_n(&bkt->key_idx[i],
- __ATOMIC_ACQUIRE);
- if (bkt->sig_current[i] == sig && key_idx != EMPTY_SLOT) {
+ if (bkt->sig_current[i] == sig && key_idx[i] != EMPTY_SLOT){
Yipeng
