u3: specify madv_random for file-backed snapshot mapping #423
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I was able to replicate the issue using v2.7 on Digital Ocean running 14 comets with 4GB of ram and 20GB of swap. Switching over to this PR I tried the same thing and still saw ~40MB/s reads.
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I ran with all the urbits running and got the below trace |
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For posterity: the actual issue was #427 |
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Thanks @mopfel-winrux, @ashelkovnykov. I think this advice is the right thing for north.bin regardless. And I'd rather not add yet-another flag to control it. But it's not pressing, and should be measured first. Leaving as a draft for now. |
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Since #402 was released, there have been isolated reports of unusually high disk i/o, specifically reads. This is unexpected behavior -- our current, best guess is the kernel's readahead logic for file-backed mappings. This PR implements the standard solution to overeager readahead (MADV_RANDOM), as a draft since the diagnosis is not confirmed, and the advice may need to be behind a flag.