Work around slow thread renaming in ClickHouse.#33
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We observed that ClickHouse can spend upwards of 80% of its cpu time in pthread_setname_np. This happens because (1) ClickHouse constantly renames its threads for debugging purposes, and (2) thread renaming is relatively expensive on illumos. Since we don't make use of this debugging path, we can work around the performance issue by bailing out of ClickHouse's setThreadName helper early. Note: this patch can't be upstreamed, so a proper long-term fix would involve adding a faster thread rename facility in illumos. Fixes oxidecomputer/customer-support#1101. h/t @wfchandler and @JustinAzoff, who found the bug and did the research.
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I think this seems ok for us, since we don't really use the name at this point. It's probably worth filing a host OS issue for the underlying problem, that setting the name appears much slower than other OS's.
Thanks to you, @wfchandler and @JustinAzoff for tracking this down!
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I wonder if you could avoid the patch entirely, but preloading a small shared object that overloads the, |
Yes, but that requires another patch. It turns out that clickhouse unsets LD_PRELOAD on start, so we'd also have to patch it for that approach anyway. |
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Oh, the irony. :-/ |
We found in oxidecomputer/customer-support#1101 that clickhouse runs a huge number of thread renames, which consure the majority of cpu time on some workloads. Since we don't need the debugging feature that this renaming supports, we patched out os-level thread renames in oxidecomputer/garbage-compactor#33. This patch updates omicron to fetch the latest clickhouse archive from s3. Note that we also introduce a hash-based suffix to the clickhouse version to represent changes in oxide-specific patches applied to the clickhouse source version. Note that we also merged a more principled fix into upstream clickhouse for this issue: ClickHouse/ClickHouse#104410. Once we upgrade to a current clickhouse version, we'll drop the patch in garbage-compactor. h/t as usual @wfchandler and @JustinAzoff for the investigation.
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We observed that ClickHouse can spend upwards of 80% of its cpu time in pthread_setname_np. This happens because (1) ClickHouse constantly renames its threads for debugging purposes, and (2) thread renaming is relatively expensive on illumos. Since we don't make use of this debugging path, we can work around the performance issue by bailing out of ClickHouse's setThreadName helper early.
Note: this patch can't be upstreamed, so a proper long-term fix would involve adding a faster thread rename facility in illumos.
Fixes https://github.com/oxidecomputer/customer-support/issues/1101. h/t @wfchandler and @JustinAzoff, who found the bug and did the research.