[Qemu-devel] [Bug 1686980] Re: qemu is very slow when adding 16, 384 virtio-scsi drives

[Daniel Berrange]

I added further instrumentation and got this profile of where the
remaining time goes

1000x drive_new 18.347secs
-> 1000x blockdev_init 18.328secs
   -> 1000x monitor_add_blk 4.515secs
      -> 1000x blk_by_name 1.545secs
      -> 1000x bdrv_find_node 2.968secs
   -> 1000x blk_new_open 13.786secs
      -> 1000x bdrv_open 13.783secs

These numbers are all increasing as we process more & more -drive args,
so there's some O(N) factor in blk_by_name, bdrv_find_node and bdrv_open

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https://bugs.launchpad.net/bugs/1686980

Title:
  qemu is very slow when adding 16,384 virtio-scsi drives

Status in QEMU:
  New

Bug description:
  qemu runs very slowly when adding many virtio-scsi drives.  I have
  attached a small reproducer shell script which demonstrates this.

  Using perf shows the following stack trace taking all the time:

      72.42%    71.15%  qemu-system-x86  qemu-system-x86_64       [.] drive_get
              |          
               --72.32%--drive_get
                         |          
                          --1.24%--__irqentry_text_start
                                    |          
                                     --1.22%--smp_apic_timer_interrupt
                                               |          
                                                
--1.00%--local_apic_timer_interrupt
                                                          |          
                                                           
--1.00%--hrtimer_interrupt
                                                                     |          
                                                                      
--0.83%--__hrtimer_run_queues
                                                                                
|          
                                                                                
 --0.64%--tick_sched_timer

      21.70%    21.34%  qemu-system-x86  qemu-system-x86_64       [.] 
blk_legacy_dinfo
              |
              ---blk_legacy_dinfo

       3.65%     3.59%  qemu-system-x86  qemu-system-x86_64       [.] blk_next
              |
              ---blk_next

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