Hyperthreading hurts server performance, say developers

Intel's Hyperthreading Technology (HT) is being blamed for server performance problems.

With both SQL Server and Citrix Terminal Server installations, HT-enabled motherboards show markedly degraded performance under heavy load. Disabling HT restores expected levels, according to reports from within the IT industry.

"Our customers were complaining about much worse performance than expected when running Citrix Terminal Server and our software on the same machine," said Peter Ibbotson, technical director of UK accounting software company Lakeview Computers.

"We've had fun and games in the past when we've enabled hyperthreading for testing and we'd seen that motherboards had started to arrive with it enabled. When we disabled hyperthreading, performance went back to normal," Ibbotson added.

Hyperthreading allows different elements of a processor to run different code at the same time, which Intel has claimed will boost chip performance and allow a CPU to process nearly twice as much information as one without hyperthreading.

Slava Ocks, a developer working on SQL Server 2005 within Microsoft, reported similar problems in a blog posting earlier this month.

"Our customers observed very interesting behaviour on high-end HT-enabled hardware. They noticed that in some cases when high load is applied SQL Server CPU usage increases significantly but SQL Server performance degrades," wrote Ocks.

Ocks then detailed testing which showed this behaviour where a system thread — in this case one cleaning out blocks of disk cache memory — is running at the same time as worker threads. "With Intel HT technology, logical processors share L1 & L2 caches. As you would guess [this] behaviour can potentially trash L1 & L2 caches," he said.

The on-chip cache exists to speed operation by keeping copies of recently accessed data where it can be accessed without recourse to main system memory — which is much slower by comparison. Where multiple threads access different parts of memory but are simultaneously processed by the chip's Hyperthreading Technology, the shared cache cannot keep up with their alternate demands and performance falls dramatically, according to analysis by Ocks and Ibbotson.

"It's ironic," said Ibbotson. "Intel had sold hyperthreading as something that gave performance gains to heavily threaded software. SQL Server is very thread-intensive, but it suffers. In fact, I've never seen performance improvement on server software with hyperthreading enabled. We recommend customers disable it when running Citrix and our software on the same server."

At the time of writing, Intel had not responded to requests for comment on these claims.

Earlier this year, Intel hyperthreading was revealed to have a security flaw where threads could find information from each other through the shared cache despite having no access to each other's memory space.