The first place where it ages an insane amount of time is simply
processing -drive options. The stack trace I see is this
(gdb) bt
#0 0x00005583b596719a in drive_get (type=type@entry=IF_NONE, bus=bus@entry=0,
unit=unit@entry=2313) at blockdev.c:223
#1 0x00005583b59679bd in drive_new (all_opts=0x5583b890e080,
block_default_type=<optimized out>) at blockdev.c:996
#2 0x00005583b5971641 in drive_init_func (opaque=<optimized out>,
opts=<optimized out>, errp=<optimized out>)
at vl.c:1154
#3 0x00005583b5c1149a in qemu_opts_foreach (list=<optimized out>,
func=0x5583b5971630 <drive_init_func>, opaque=0x5583b9980030, errp=0x0) at
util/qemu-option.c:1114
#4 0x00005583b5830d30 in main (argc=<optimized out>, argv=<optimized out>,
envp=<optimized out>) at vl.c:4499
We're iterating over every -drive option. Now because we're using if=none, and
thus unit==0, line 996 of blockdev.c looks calling drive_get() until we find a
matching drive, in order to identify the unit number. So we have a loop over
every drive, calling drive_new which loops over every drive calling drive_get
which loops over every drive. So about O(N*N*N)
--
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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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