[PROTON-2353] Client becomes cpu bound in pn_buffer_rotate() on session flow control

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: proton-c-0.33.0, proton-c-0.35.0, proton-c-0.34.0, proton-c-0.36.0
• Component/s: proton-c
• Assignee: Unassigned
• Reporter: Ken Giusti
• Created: 29/Mar/21 12:50
• Updated: 14/Feb/22 18:01

Description

While testing various AMQP session size limitations on the Qpid dispatch router I was able to cause a proton-c client to pin the CPU at 100% while in pn_buffer_rotate().

This appears to be caused by lowering the incoming session window to a point where session flow control activates.  In the case of my reproducer I'm setting the window size to approx 600 16K frames ~9.8MB.  The client is sending ~1MB messages with a credit window of 250.

 

See the attached flamegraphs of the sending client:

First - this is baseline, no session limit (default provided by proton):

big-session.svg

 

This flamegraph shows the client's behavior with a session limit of 9.8MB:

small-session.svg

 

See comment for reproducer.

Issue Links

• is depended upon by: PROTON-2321 Add benchmark for pn_data_t initialization (PROTON-2229)