The Design and Implementation of the Anykernel and Rump Kernels

181
25
remote (unix domain)
local
20
time (s)
15
10
5
0
0 1 2 3 4 5 6 7 8
copyin/out pairs
Figure 3.37: Local vs. Remote system call overhead. Thecostofremote
system calls is dominated by the amount of client-kernel roundtrips necessary due
to copying data in and out. For local clients the cost of a system call is virtually
independent of the amount of copies in and out.
The straightforward optimization which does not involve modifying the host system
is to decrease the number of remote copyin/out requests required for completing a
syscall request. This decrease can be reached in a fairly straightforward manner by
augmenting the syscall definitions and pre-arranging parameters so that pre-known
copyin/out I/O can be avoided. Possible options are piggy-backing the data copy
as part of syscall request/response, or by using interprocess shared memory in case
the client and server are on the same machine. For example, the open() syscall
will, barring an early error, always copy in the pathname string. We can make the
syscall code set things up so that the pathname copyin is immediately satisfied with
a local copy operation instead of a remote request and the associated round trip
delay.