Re: [PATCH 3/7] net/bonding: change mbuf pool and ring allocation

Hi, Sanford,

在 2021/12/21 0:47, Sanford, Robert 写道:

Hello Connor,

Please see responses inline.

On 12/17/21, 10:44 PM, "Min Hu (Connor)" [off-list ref] wrote:

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When the number of used tx-descs (0..255) + number of mbufs in the
cache (0..47) reaches 257, then allocation fails.

If I understand the LACP tx-burst code correctly, it would be
worse if nb_tx_queues > 1, because (assuming multiple tx-cores)
any queue/lcore could xmit an LACPDU. Thus, up to nb_tx_queues *
47 mbufs could be cached, and not accessible from tx_machine().

You would not see this problem if the app xmits other (non-LACP)
mbufs on a regular basis, to expedite the clean-up of tx-descs
including LACPDU mbufs (unless nb_tx_queues tx-core caches
could hold all LACPDU mbufs).

I think, we could not see this problem only because the mempool can
offer much more mbufs than cache size on no-LACP circumstance.

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If we make mempool's cache size 0, then allocation will not fail.

How about enlarge the size of mempool, i.e., up to 4096 ? I think
it can also avoid this bug.

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A mempool cache for LACPDUs does not offer much additional speed:
during alloc, the intr thread does not have default mempool caches

Why? as I know, all the core has its own default mempool caches ?

These are private mbuf pools that we use *only* for LACPDUs, *one*
mbuf per second, at most. (When LACP link peer selects long timeouts,
we get/put one mbuf every 30 seconds.)

There is *NO* benefit for the consumer thread (interrupt thread
executing tx_machine()) to have caches on per-slave LACPDU pools.
The interrupt thread is a control thread, i.e., a non-EAL thread.
Its lcore_id is LCORE_ID_ANY, so it has no "default cache" in any
mempool.

Well, sorry, I forgot that interrupt thread is non-EAL thread.

There is little or no benefit for active data-plane threads to have
caches on per-slave LACPDU pools, because on each pool, the producer
thread puts back, at most, one mbuf per second. There is not much
contention with the consumer (interrupt thread).

I contend that caches are not necessary for these private LACPDU

I agree with you.

mbuf pools, but just waste RAM and CPU-cache. If we still insist
on creating them *with* caches, then we should add at least
(cache-size x 1.5 x nb-tx-queues) mbufs per pool.

  

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Q: Why reserve one additional slot in the rx and tx rings?

A: rte_ring_create() requires the ring size N, to be a power of 2,
but it can only store N-1 items. Thus, if we want to store X items,

Hi, Robert, could you describe it for me?
I cannot understand why it
"only store N -1 items". I check the source code, It writes:
"The real usable ring size is *count-1* instead of *count* to
differentiate a free ring from an empty ring."
But I still can not get what it wrote.

I believe there is a mistake in the ring comments (in 3 places).
It would be better if they replace "free" with "full":
"... to differentiate a *full* ring from an empty ring."

Well, I still can not understand it. I think the ring size is N, it
should store N items, why "N - 1" items.?
Hope for your description, thanks.

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we need to ask for (at least) X+1. Original code fails when the real
desired size is a power of 2, because in such a case, align32pow2
does not round up.

For example, say we want a ring to hold 4:

      rte_ring_create(... rte_align32pow2(4) ...)

rte_align32pow2(4) returns 4, and we end up with a ring that only
stores 3 items.

      rte_ring_create(... rte_align32pow2(4+1) ...)

rte_align32pow2(5) returns 8, and we end up with a ring that
stores up to 7 items, more than we need, but acceptable.

To fix the bug, how about just setting the flags "RING_F_EXACT_SZ"

Yes, this is a good idea. I will look for examples or test code that
use this flag.

Yes, if fixed, ILGM.

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Regards,
Robert Sanford