Hi, On Wed, Apr 17, 2024 at 6:51 PM Alexander Aring <aahringo@xxxxxxxxxx> wrote: > > Hi Mathieu, > > On Tue, Apr 16, 2024 at 10:49 AM Mathieu Desnoyers > <mathieu.desnoyers@xxxxxxxxxxxx> wrote: > > > > On 2024-04-16 10:41, Alexander Aring wrote: > > > Hi all, > > > > > > I have an idea about using the DMA rings of a networking card as a > > > tracing buffer. > > > > The first thing I'd be worried about is overhead caused by > > false-sharing if you trace a large SMP/many-core system all > > into the same global NIC buffer. The cache lines will bounce > > left and right which is bad performance-wise. > > > > modern NICs have several tx and rx dma rings. There is an ethtool > command to query the number: > > sudo ethtool -g $IFACE > > so it depends on the hardware if you can pin one or more rings on a > per-cpu buffer basis. > > I hope that somehow helps to make this idea less worrisome. > What I can think about that makes it worrisome is NUMA. In case of an E-PCI connected NIC, the NICs buffer associated with a core needs to be in the same NUMA node. You need to set up your system right, even with 2 nics or more, to make it work "better". - Alex
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