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Subject: mbuf fast free optimization idea
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Date: Sat, 26 Aug 2023 12:04:18 +0200
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Thread-Topic: mbuf fast free optimization idea
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From: =?iso-8859-1?Q?Morten_Br=F8rup?= <mb@smartsharesystems.com>
To: <olivier.matz@6wind.com>, <thomas@monjalon.net>,
 "Shahaf Shuler" <shahafs@nvidia.com>
Cc: <dev@dpdk.org>
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When RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE is used, the usage pattern of =
some of the mbuf fields differs:

- The "pool" pointer becomes rarely used. (Only one packet per burst.)
- The "next" pointer becomes unused.
- The "tx_offload" field is still used. (Set by application, read by =
driver.)

This means that the 2nd cache line in the mbuf now only contains one hot =
field, "tx_offload".

So, if we swap the "tx_offload" field (in the 2nd cache line) with the =
"pool" field (in the 1st cache line), the 2nd cache line becomes rarely =
used.

This should provide a performance improvement for applications (using =
RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE) when under cache pressure.

PS: Back in the days, the mbuf description said that the 1st cache line =
was for RX purposes, and the 2nd cache line was for TX purposes. This is =
not true anymore, so let's not limit out thinking by that (obsolete) =
design.


Med venlig hilsen / Kind regards,
-Morten Br=F8rup