[stgt] [PATCH RFC] tgtd: send/recv iSCSI PDUs in worker threads

[Hitoshi Mitake]

At Tue, 05 Nov 2013 23:28:45 +0900 (JST),
FUJITA Tomonori wrote:
> 
> On Mon,  4 Nov 2013 14:13:16 +0900
> Hitoshi Mitake <mitake.hitoshi at gmail.com> wrote:
> 
> > From: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
> > 
> > Current tgtd sends and receives iSCSI PDUs in its main event
> > loop. This design can cause bottleneck when many iSCSI clients connect
> > to single tgtd process. For example, we need multiple tgtd processes
> > for utilizing fast network like 10 GbE because typical single
> > processor core isn't fast enough for processing bunch of requests.
> > 
> > This patch lets tgtd send/receive iSCSI PDUs and check digests in its
> > worker threads. After applying this patch, the bottleneck in the main
> > event loop is removed and the performance is improved.
> > 
> > The improvement can be seen even if tgtd and iSCSI initiator are
> > running on a single host. Below is a snippet of fio result on my
> > laptop. The workload is 128MB random RW. Backingstore is sheepdog.
> > 
> > original tgtd:
> >   read : io=65392KB, bw=4445.2KB/s, iops=1111, runt= 14711msec
> >   write: io=65680KB, bw=4464.8KB/s, iops=1116, runt= 14711msec
> > 
> > tgtd with this patch:
> >   read : io=65392KB, bw=5098.9KB/s, iops=1274, runt= 12825msec
> >   write: io=65680KB, bw=5121.3KB/s, iops=1280, runt= 12825msec
> > 
> > This change will be more effective when a number of iSCSI clients
> > increases. I'd like to hear your comments on this change.
> > 
> > Signed-off-by: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
> > ---
> >  usr/iscsi/iscsi_tcp.c | 291 +++++++++++++++++++++++++++++++++++++++++++++++---
> >  usr/iscsi/iscsid.c    |  61 +++++++----
> >  usr/iscsi/iscsid.h    |   4 +
> >  3 files changed, 322 insertions(+), 34 deletions(-)
> 
> This change doesn't affect our complicated logic to handle outstanding
> commands with tcp disconnection (e.g. conn_close() in conn.c)?

I think it doesn't affect the closing logic. Because any procedure
other than send/recv and digest checking are not delegated to worker
threads. Task queuing, connection closing, etc are done in main
thread even now.

Thanks,
Hitoshi

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