child jitter vs parent shaping vs child class-default on 871 and 1841

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I was wondering in anyone has came across the same problem:
Implementing parent policy map with shaping on class-default on Cisco
871 or 1841 will cause higher jitter on child traffic class even if it
is LLQ. I guess it should be the same behavior on other Cisco access

policy-map Child
 class voip
  priority 500
   police 500000
 class data
  bandwidth 400

policy-map Parent
 class class-default
  shape average 2000000
  service-policy Child

Interface FastEthernet 4
 service-policy output Parent

After trying different burst size parameters on parent shaping I found
out that jitter can be minimized by reducing parent shaping burst
size. By minimizing this parameter child policy-map reduced default
allocated bandwidth to class-default - 15% or even less. Which is not
what Cisco tells - 75% + 25%.
I know that Cisco does not recommends to tune shaping burst size, but
I could not find any other way to reduce jitter. IOS Version:


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