Evgeniy Polyakov wrote:
Couple of numbers...
Remapping of the physical page took about 25-50% less time than 1500
bytes copying using memcpy().
And 15 times faster just after reboot, i.e. without anything in the
cache.
CPU is Xeon with HT enabled:
cpu family : 15
model : 2
model name : Intel(R) Xeon(TM) CPU 2.40GHz
stepping : 7
cpu MHz : 800.384
1.
packet_mmap_test: 1000 remaps took 1495 usec.
packet_mmap_test: 1000 copyings took 1988 usec.
2.
packet_mmap_test: 1000 remaps took 1406 usec.
packet_mmap_test: 1000 copyings took 2613 usec.
3. And just after reboot, when there is nothing in cache:
packet_mmap_test: 1000 remaps took 1387 usec.
packet_mmap_test: 1000 copyings took 20173 usec.
4. Yet another "just after reboot":
packet_mmap_test: 1000 remaps took 1295 usec.
packet_mmap_test: 1000 copyings took 14889 usec.
Above copying is being done using arbitrary kernel virtual address
as source address and with PAGE_SIZE addition to it before each
memcpy().
I'm almost convinced that remap can brake even with ring buffer at 1500
sizes. However we're talking about total per packet overhead here. At some
point you have to _unmap_ the thing once user-space app is done with it.
With ring buffer it's "memcpy()/kfree_skb()" where with mmap it's
"remap()/unmap()/kfree_skb()".
Max
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