netdev - Re: [PATCH 01/10 net-next] net/mlx5e: RX, Remove RX page-cache

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Date:   Sun, 20 Oct 2019 07:29:07 +0000
From:   Tariq Toukan <tariqt@...lanox.com>
To:     Jonathan Lemon <jonathan.lemon@...il.com>,
        Saeed Mahameed <saeedm@...lanox.com>
CC:     "ilias.apalodimas@...aro.org" <ilias.apalodimas@...aro.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        "brouer@...hat.com" <brouer@...hat.com>,
        Netdev <netdev@...r.kernel.org>, kernel-team <kernel-team@...com>
Subject: Re: [PATCH 01/10 net-next] net/mlx5e: RX, Remove RX page-cache



On 10/19/2019 3:10 AM, Jonathan Lemon wrote:
> I was running the updated patches on machines with various workloads, and
> have a bunch of different results.
> 
> For the following numbers,
>    Effective = hit / (hit + empty + stall) * 100
> 
> In other words, show the hit rate for for every trip to the cache,
> and the cache full stat is ignored.
> 
> On a webserver:
> 
> [web] # ./eff
> ('rx_pool_cache_hit:', '360127643')
> ('rx_pool_cache_full:', '0')
> ('rx_pool_cache_empty:', '6455735977')
> ('rx_pool_ring_produce:', '474958')
> ('rx_pool_ring_consume:', '0')
> ('rx_pool_ring_return:', '474958')
> ('rx_pool_flush:', '144')
> ('rx_pool_node_change:', '0')
> cache effectiveness:  5.28
> 
> On a proxygen:
> # ethtool -S eth0 | grep rx_pool
>       rx_pool_cache_hit: 1646798
>       rx_pool_cache_full: 0
>       rx_pool_cache_empty: 15723566
>       rx_pool_ring_produce: 474958
>       rx_pool_ring_consume: 0
>       rx_pool_ring_return: 474958
>       rx_pool_flush: 144
>       rx_pool_node_change: 0
> cache effectiveness:  9.48
> 
> On both of these, only pages with refcount = 1 are being kept.
> 
> 
> I changed things around in the page pool so:
> 
> 1) the cache behaves like a ring instead of a stack, this
>     sacrifices temporal locality.
> 
> 2) it caches all pages returned regardless of refcount, but
>     only returns pages with refcount=1.
> 
> This is the same behavior as the mlx5 cache.  Some gains
> would come about if the sojourn time though the cache is
> greater than the lifetime of the page usage by the networking
> stack, as it provides a fixed working set of mapped pages.
> 
> On the web server, this is a net loss:
> [web] # ./eff
> ('rx_pool_cache_hit:', '6052662')
> ('rx_pool_cache_full:', '156355415')
> ('rx_pool_cache_empty:', '409600')
> ('rx_pool_cache_stall:', '302787473')
> ('rx_pool_ring_produce:', '156633847')
> ('rx_pool_ring_consume:', '9925520')
> ('rx_pool_ring_return:', '278788')
> ('rx_pool_flush:', '96')
> ('rx_pool_node_change:', '0')
> cache effectiveness:  1.95720846778
> 
> For proxygen on the other hand, it's a win:
> [proxy] # ./eff
> ('rx_pool_cache_hit:', '69235177')
> ('rx_pool_cache_full:', '35404387')
> ('rx_pool_cache_empty:', '460800')
> ('rx_pool_cache_stall:', '42932530')
> ('rx_pool_ring_produce:', '35717618')
> ('rx_pool_ring_consume:', '27879469')
> ('rx_pool_ring_return:', '404800')
> ('rx_pool_flush:', '108')
> ('rx_pool_node_change:', '0')
> cache effectiveness:  61.4721608624
> 
> So the correct behavior isn't quite clear cut here - caching a
> working set of mapped pages is beneficial in spite of the HOL
> blocking stalls for some workloads, but I'm sure that it wouldn't
> be too difficult to exceed the WS size.
> 
> Thoughts?
> 

We have a WIP in which we avoid the HOL block, by having pages returned 
to the available-queue only when their refcnt reaches back to 1. This 
requires catching this case in the page/skb release path.

See:
https://github.com/xdp-project/xdp-project/tree/master/areas/mem