David O'Brien wrote: > On Mon, Aug 25, 2003 at 08:27:46AM -0600, Scott Long wrote: > >>Since HTT can lead to performance degradation in some (many?) cases, >>the second logical CPU's are halted by default. They are enabled, >>however, in order for interrupt routing to work right. Work is ongoing >>to make an HTT-aware scheduler, and make the enabling of the logical >>cores optional. > > > I've heard this several times and don't doubt it, but it would be nice to > know more about the issue. What type of cases? What benchmarks have > been run showing this? Well, I haven't actually seen any case where there was a performance gain instead of degradation. There are two problems with HTT. First, L1/L2 cache issues. Second, the virtual CPUs are not independent, and there are many cases where instructions in one virtual CPU stall the other. So take, for example, the case of a userland application on CPU0 stalling the kernel on CPU1. The case where HTT presents gains are for applications compiled in a way to maximize HTT benefits and minimize HTT stalling. It would be perhaps be best to restrict HTT usage to threads of a same application, and avoid them at all when in kernel. Intel disagrees, of course, and I haven't been a low level person for many, many years, so YMMV. :-) -- Daniel C. Sobral (8-DCS) Gerencia de Operacoes Divisao de Comunicacao de Dados Coordenacao de Seguranca VIVO Centro Oeste Norte Fones: 55-61-313-7654/Cel: 55-61-9618-0904 E-mail: Daniel.Capo_at_tco.net.br Daniel.Sobral_at_tcoip.com.br dcs_at_tcoip.com.br Outros: dcs_at_newsguy.com dcs_at_freebsd.org capo_at_notorious.bsdconspiracy.net A raccoon tangled with a 23,000 volt line today. The results blacked out 1400 homes and, of course, one raccoon. -- Steel City NewsReceived on Mon Aug 25 2003 - 08:14:30 UTC
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