Re: Integration of SCST in the mainstream Linux kernel
- From: "Bart Van Assche" <bart.vanassche@xxxxxxxxx>
- Date: Wed, 20 Feb 2008 09:41:24 +0100
On Feb 20, 2008 8:34 AM, Erez Zilber <erezz@xxxxxxxxxxxx> wrote:
Bart Van Assche wrote:
Or: data sent during the first burst is not transferred via one-sided
remote memory reads or writes but via two-sided send/receive
operations. At least on my setup, these operations are as fast as
one-sided remote memory reads or writes. As an example, I obtained the
following numbers on my setup (SDR 4x network);
ib_write_bw: 933 MB/s.
ib_read_bw: 905 MB/s.
ib_send_bw: 931 MB/s.
According to these numbers one can think that you don't need RDMA at
all, just send iSCSI PDUs over IB.
Sorry, but you are misinterpreting what I wrote.
The benchmarks that you use are
synthetic IB benchmarks that are not equivalent to iSCSI over iSER. They
just send IB packets. I'm not surprised that you got more or less the
same performance because, AFAIK, ib_send_bw doesn't copy data (unlike
iSCSI that has to copy data that is sent/received without RDMA).
I agree that ib_write_bw / ib_read_bw / ib_send_bw performance results
are not equivalent to iSCSI over iSER. The reason that I included
these performance results was to illustrate that two-sided data
transfers over IB are about as fast as one-sided data transfers.
When you use RDMA with iSCSI (i.e. iSER), you don't need to create iSCSI
PDUs and process them. The CPU is not busy as it is with iSCSI over TCP
because no data copies are required. Another advantage is that you don't
need header/data digest because the IB HW does that.
As far as I know, when using iSER, the FirstBurstLength bytes of data
are sent via two-sided data transfers, and there is no CPU
intervention required to transfer the data itself over the IB network.
Bart Van Assche.
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- References:
- Re: Integration of SCST in the mainstream Linux kernel
- From: Erez Zilber
- Re: Integration of SCST in the mainstream Linux kernel
- From: Bart Van Assche
- Re: Integration of SCST in the mainstream Linux kernel
- From: Erez Zilber
- Re: Integration of SCST in the mainstream Linux kernel
- From: Bart Van Assche
- Re: Integration of SCST in the mainstream Linux kernel
- From: Erez Zilber
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