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

[Daniel Berrange]

I instrumented drive_new to time how long 1000 creations took with
current code:

1000 drive_new() in 0 secs
1000 drive_new() in 2 secs
1000 drive_new() in 18 secs
1000 drive_new() in 61 secs

As a quick hack you can just disable the drive_get() calls when if=none.
They're mostly just used to fill in default unit_id, but that's not
really required for if=none. That said, if no id= parameter is set, then
the code does expect unit_id to be valid, so not sure how to fully fix
that.

Anyway, with this hack applied it is much faster, but there is still
some kind of N*N complexity going on, because drive_new() gets slower &
slower as each drive is created - just not nearly as badly as before.

1000 drive_new() in 0 secs
1000 drive_new() in 0 secs
1000 drive_new() in 0 secs
1000 drive_new() in 1 secs
1000 drive_new() in 1 secs
1000 drive_new() in 1 secs
1000 drive_new() in 2 secs
1000 drive_new() in 2 secs
1000 drive_new() in 2 secs
1000 drive_new() in 4 secs
1000 drive_new() in 4 secs
1000 drive_new() in 6 secs
1000 drive_new() in 8 secs
1000 drive_new() in 8 secs


** Patch added: "Hack to avoid drive_new() when if=none"
   
https://bugs.launchpad.net/qemu/+bug/1686980/+attachment/4869119/+files/skip-drive-get-if-none.patch

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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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