Re: io_request_lock patch?

Mike Anderson
2001-07-10T23:05:55Z

The call to do_aic7xxx_isr appears that you are running the aic7xxx_old.c
code. This driver is using the io_request_lock to protect internal data.
The newer aic driver has its own lock. This is related to previous
comments by Jens and Eric about lower level use of this lock.

I would like to know why the request_freelist is going empty? Having
__get_request_wait being called alot would appear to be not optimal.

-Mike

Dipankar Sarma [dipankar@sequent.com] wrote:
> Hi Jens,
>
> In article <20010709214453.U16505@suse.de> you wrote:
> > On Mon, Jul 09 2001, Jonathan Lahr wrote:
> > It's also interesting to take a look at _why_ there's contention on the
> > io_request_lock. And fix those up first.
>
> > --
> > Jens Axboe
>
> Here are some lockmeter outputs for tiobench
> (http://sourceforge.net/projects/tiobench), a simple benchmark
> that we tried on ext2 filesystem. 4 concurrent threads doing
> random/sequential read/write on 10MB files on a 4-way pIII 700MHz
> machine with 1MB L2 cache -
>
> SPINLOCKS HOLD WAIT
> UTIL CON MEAN( MAX ) MEAN( MAX )(% CPU) TOTAL NOWAIT SPIN RJECT NAME
>
> 2.9% 26.7% 7.4us( 706us) 72us( 920us)( 1.9%) 1557496 73.3% 26.7% 0% io_request_lock
> 0.00% 34.9% 0.5us( 2.8us) 63us( 839us)(0.04%) 29478 65.1% 34.9% 0% __get_request_wait+0x98
> 2.6% 4.7% 17us( 706us) 69us( 740us)(0.13%) 617741 95.3% 4.7% 0% __make_request+0x110
> 0.07% 60.2% 0.5us( 4.0us) 72us( 920us)( 1.7%) 610820 39.8% 60.2% 0% blk_get_queue+0x10
> 0.09% 2.9% 6.6us( 55us) 102us( 746us)(0.01%) 55327 97.1% 2.9% 0% do_aic7xxx_isr+0x24
> 0.00% 3.7% 0.3us( 22us) 29us( 569us)(0.00%) 22602 96.3% 3.7% 0% generic_unplug_device+0x10
> 0.02% 4.9% 1.3us( 27us) 54us( 621us)(0.01%) 55382 95.1% 4.9% 0% scsi_dispatch_cmd+0x12c
> 0.02% 1.3% 1.2us( 8.0us) 23us( 554us)(0.00%) 55382 98.7% 1.3% 0% scsi_old_done+0x5b8
> 0.04% 3.2% 2.8us( 31us) 200us( 734us)(0.02%) 55382 96.8% 3.2% 0% scsi_queue_next_request+0x18
> 0.02% 1.4% 1.1us( 7.8us) 46us( 638us)(0.00%) 55382 98.6% 1.4% 0% scsi_request_fn+0x350
>
> 1557496*26.7%*72us makes it about 30 seconds of time waiting for
> io_request_lock. That is nearly one-third of the total system time
> (about 98 seconds). As number of CPUs increase, this will likely
> worsen.
>
> It also seems that __make_request() holds the lock for the largest
> amount of time. This hold time isn't likely to change significantly
> for a per-queue lock, but atleast it will not affect queueing i/o
> requests to other devices. Besides, I am not sure if blk_get_queue()
> really needs to grab the io_request_lock. blk_dev[] entries aren't
> likely to be updated in an open device and hence it should be
> safe to look up the queue of an open device. For mutual
> exclusion in the device-specific queue() function, it might be
> better to leave it to the driver instead of forcing the mutual
> exclusion. For example, a driver might want to use a reader/writer
> lock to lookup its device table for the queue. It also might make sense to
> have separate mutual exclusion mechanism for block device
> and scsi device level queues.
>
> Thanks
> Dipankar
> --
> Dipankar Sarma <dipankar@sequent.com> Project: http://lse.sourceforge.net
> Linux Technology Center, IBM Software Lab, Bangalore, India.
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--
Michael Anderson
mike.anderson@us.ibm.com