On 02/06/15 14:31, Ananyev, Konstantin wrote: > Hi Zoltan, > >> -----Original Message----- >> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Zoltan Kiss >> Sent: Monday, June 01, 2015 5:16 PM >> To: dev at dpdk.org >> Subject: Re: [dpdk-dev] [PATCH] ixgbe: fix checking for tx_free_thresh >> >> Hi, >> >> Anyone would like to review this patch? Venky sent a NAK, but I've >> explained to him why it is a bug. > > > Well, I think Venky is right here. I think the comments above rte_eth_tx_burst() definition are quite clear about what tx_free_thresh means, e1000 and i40e use it that way, but not ixgbe.
> Indeed that fix, will cause more often unsuccessful checks for DD bits and > might cause a > slowdown for TX fast-path. Not if the applications set tx_free_thresh according to the definition of this value. But we can change the default value from 32 to something higher, e.g I'm using nb_desc/2, and it works out well. > Anyway, with current PMD implementation, you can't guarantee that at any > moment > TX queue wouldn't use more than tx_free_thresh mbufs. > There could be situations (low speed, or link is down for some short period, > etc), when > much more than tx_free_thresh TXDs are in use and none of them could be freed > by HW right now. > So your app better be prepared, that up to (nb_tx_desc * num_of_TX_queues) > could be in use > by TX path at any given moment. > > Though yes, there is an inconsistency how different ixgbe TX functions treat > tx_conf->tx_free_thresh parameter. > That probably creates wrong expectations and confusion. Yes, ixgbe_xmit_pkts() use it the way it's defined, this two function doesn't. > We might try to unify it's usage one way or another, but I personally don't > see much point in it. > After all, tx_free_tresh seems like a driver internal choice (based on the > nb_tx_desc and other parameters). > So I think a better way would be: > 1. Deprecate tx_conf->tx_free_thresh (and remove it in later releases) and > make > each driver to use what it thinks would be the best value. But how does the driver knows what's the best for the applications traffic pattern? I think it's better to leave the possibility for the app to fine tune it. In the meantime we can improve the default selection as well, as I suggested above. > 2. As you suggested in another mail, introduce an new function: > uint16_t rte_eth_tx_free_pkts(port_id, queue_id, nb_to_free). > That would give upper layer a better control of memory usage, and might be > called by the upper layer at idle time, > so further tx_burst, don't need to spend time on freeing TXDs/packets. I agree. > > Konstantin > > >> >> Regards, >> >> Zoltan >> >> On 27/05/15 21:12, Zoltan Kiss wrote: >>> This check doesn't do what's required by rte_eth_tx_burst: >>> "When the number of previously sent packets reached the "minimum transmit >>> packets to free" threshold" >>> >>> This can cause problems when txq->tx_free_thresh + [number of elements in >>> the >>> pool] < txq->nb_tx_desc. >>> >>> Signed-off-by: Zoltan Kiss <zoltan.kiss at linaro.org> >>> --- >>> drivers/net/ixgbe/ixgbe_rxtx.c | 4 ++-- >>> drivers/net/ixgbe/ixgbe_rxtx_vec.c | 2 +- >>> 2 files changed, 3 insertions(+), 3 deletions(-) >>> >>> diff --git a/drivers/net/ixgbe/ixgbe_rxtx.c b/drivers/net/ixgbe/ixgbe_rxtx.c >>> index 4f9ab22..b70ed8c 100644 >>> --- a/drivers/net/ixgbe/ixgbe_rxtx.c >>> +++ b/drivers/net/ixgbe/ixgbe_rxtx.c >>> @@ -250,10 +250,10 @@ tx_xmit_pkts(void *tx_queue, struct rte_mbuf >>> **tx_pkts, >>> >>> /* >>> * Begin scanning the H/W ring for done descriptors when the >>> - * number of available descriptors drops below tx_free_thresh. For >>> + * number of in flight descriptors reaches tx_free_thresh. For >>> * each done descriptor, free the associated buffer. >>> */ >>> - if (txq->nb_tx_free < txq->tx_free_thresh) >>> + if ((txq->nb_tx_desc - txq->nb_tx_free) > txq->tx_free_thresh) >>> ixgbe_tx_free_bufs(txq); >>> >>> /* Only use descriptors that are available */ >>> diff --git a/drivers/net/ixgbe/ixgbe_rxtx_vec.c >>> b/drivers/net/ixgbe/ixgbe_rxtx_vec.c >>> index abd10f6..f91c698 100644 >>> --- a/drivers/net/ixgbe/ixgbe_rxtx_vec.c >>> +++ b/drivers/net/ixgbe/ixgbe_rxtx_vec.c >>> @@ -598,7 +598,7 @@ ixgbe_xmit_pkts_vec(void *tx_queue, struct rte_mbuf >>> **tx_pkts, >>> if (unlikely(nb_pkts > RTE_IXGBE_VPMD_TX_BURST)) >>> nb_pkts = RTE_IXGBE_VPMD_TX_BURST; >>> >>> - if (txq->nb_tx_free < txq->tx_free_thresh) >>> + if ((txq->nb_tx_desc - txq->nb_tx_free) > txq->tx_free_thresh) >>> ixgbe_tx_free_bufs(txq); >>> >>> nb_commit = nb_pkts = (uint16_t)RTE_MIN(txq->nb_tx_free, nb_pkts); >>>
