Add an option to fix the message size

Test the question, is it the data volume, or the number of messages, that limits the wall clock time.

added 1 commit

• 46a6caa0 - Fix a memory allocation bug (*nlogs) and stop allocating all the buffer memory upfront

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added 2 commits

• 41f45fa1 - 1 commit from branch master
• 2ffa242b - Merge branch 'master' into fixed-size

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added 22 commits

• 2ffa242b...615283c3 - 21 commits from branch master
• d7e82b66 - Merge branch 'master' into fixed-size

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merged