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Alan Hargreaves's Weblog

DTrace Lab in the performance track at CEC

Alan Hargreaves
Senior Principal Technical Support Engineer

We have just been helping out in the DTrace lab for the performance track at CEC. On having a look at exercise 1, we suggested to the attendees that they attempt the other exercises and come back to this one. The exercise was:

Enhance /usr/demo/dtrace/iosnoop to be able to detect reads that are satisfied by the filesystem cache (UFS: pagecache).

We then went off to write our own versions of an answer to the problem.I started from scratch, and (referring back to a previous blog where I did some digging into this code), came up with the following:

#!/usr/sbin/dtrace -s
#pragma D option quiet
BEGIN { printf("Collecting ...\\n"); }
syscall::read\*:entry {
self->interest = 1;
self->file = fds[arg0].fi_name;
self->phys = 0;
}
/\* we only increment this kstat if we have to go to disk, from a filesystem \*/
sysinfo:::bread /self->interest/ { self->phys = 1;}
syscall::read\*:return /self->interest/ {
this->str = self->phys
? "Physical reads"
: "Cache Reads";
@[this->str, self->file] = count();
self->interest = 0;
}
END { printa("%@6d %s %s\\n", @); }

On running this you get output like:

   ...
209 Cache Reads jaxrpc-impl.jar
211 Cache Reads appserv-admin.jar
311 Cache Reads xalan.jar
354 Cache Reads ttysrch
372 Cache Reads jaxr-impl.jar
373 Cache Reads auxv
500 Cache Reads appserv-assemblytool.jar
1216 Cache Reads
1324 Cache Reads appserv-rt.jar
1392 Cache Reads clone@0:ptm
83181 Cache Reads psinfo

Of course by the time I finished writing this, pretty much everything in the only ufs filesystem on the box had been cached :)

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