Re: io_request_lock patch?

[Mike Anderson <mike.anderson@us.ibm.com>]

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