[stgt] [PATCH RFC v2] tgtd: send/recv iSCSI PDUs in worker threads

[Hitoshi Mitake]

At Sun, 24 Nov 2013 23:06:02 +0900,
Hitoshi Mitake wrote:
> 
> At Fri, 22 Nov 2013 09:29:49 +0900,
> FUJITA Tomonori wrote:
> > 
> > On Sun, 10 Nov 2013 17:49:07 +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>
> > > ---
> > > 
> > > v2:
> > >  - correct handling of connection closing based on a reference count of an iSCSI
> > >    connection
> > >  - a silly bug in iscsi_tcp_init() introduced in the previous patch is removed
> > > 
> > >  usr/iscsi/conn.c      |  12 ++-
> > >  usr/iscsi/iscsi_tcp.c | 293 +++++++++++++++++++++++++++++++++++++++++++++++---
> > >  usr/iscsi/iscsid.c    |  61 +++++++----
> > >  usr/iscsi/iscsid.h    |   8 +-
> > >  4 files changed, 334 insertions(+), 40 deletions(-)
> > 
> > This can handle a single iscsi session having multiple tcp
> > connections? For example, if iscsi_scsi_cmd_rx_start() is called by
> > multiple threads, conn->session->cmd_list is accessed by them without
> > a lock?
> 
> As you say, some functions which manipulate data structures related to
> session can be called by multiple threads in an unsafe manner, sorry.
> 
> I'll post v3 later. In the next version, iscsi_task_rx_start() and
> iscsi_task_tx_start() are executed in a serialized manner. AFAIK,
> other part of multithreaded code don't touch the data structures.

BTW, I'll be able to execute a benchmark on 10 GbE environment next
week. I'd like to post the result soon.

Thanks,
Hitoshi
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