Mach number which was also unit-first, and may have influenced the use of the term Mach. In the decade preceding faster-than-sound human flight, aeronautical engineers referred to the speed of sound as "Mach's number", never "Mach 1". Mach number is useful because the fluid behaves in a similar manner at a given Mach number, regardless of other variables. As modeled in the International Standard Atmosphere, dry air at mean sea level, standard temperature of , the speed of sound is . The speed of sound is not a constant; in a gas, it increases as the absolute temperature increases, and since atmospheric
Mach (kernel) it would take just under 500μs. When Mach was first being seriously used in the 2.x versions, performance was slower than traditional monolithic operating systems, perhaps as much as 25%. This cost was not considered particularly worrying, however, because the system was also offering multi-processor support and easy portability. Many felt this was an expected and acceptable cost to pay. When Mach 3 attempted to move most of the operating system into user-space, the overhead became higher still: benchmarks between Mach and Ultrix on a MIPS R3000 showed a performance hit as great as 67% on some workloads. For example,