[bug #66127] [Bash x86] Increased memory bandwidth due to background commands

[korantli]

URL:
  <https://savannah.gnu.org/bugs/?66127>

                 Summary: [Bash x86] Increased memory bandwidth due to
background commands
                   Group: The GNU Bourne-Again SHell
               Submitter: korantli
               Submitted: Mon 26 Aug 2024 03:10:18 AM UTC
                Category: None
                Severity: 3 - Normal
              Item Group: None
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any


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Follow-up Comments:


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Date: Mon 26 Aug 2024 03:10:18 AM UTC By: korantli <korantli>
Hey guys，we found a strange issue about bash

In bash version: v4.2.46\v4.4.20\v5.2, we found following issue.

When we run the script blow：
    #! /bin/bash
    while :; do
        echo "test" &                                                         
                                              
        # wait $!
        sleep 1
    done

We found that background commands cause memory bandwidth to increase with
runtime by intel's tool pcm-memory（https://github.com/intel/pcm）. 

When first start running the script, the bandwidth is about 50-100MB/s. But
after 3 days, it increases to 1000-1500MB/s.

Besides, we found the script's RSS(watched by pidstat) will continue to
increase.

We found when issue occurred, the script's
_int_free\_int_malloc\malloc_consolidate is first 3 top hot perf events
collected by "perf record -e mem-loads -e mem-stores -g -p  xxx -- sleep 10".

However when first start running script, they are not the hottest. 

We believe the these mem events is the key reason why memory bandwidth
increasing. However, we can't find out why bash do it.

Also, we found "(echo "test" &)" will fix this issue.

It will be so grateful if anyone could help~







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