Shailabh Nagar wrote:
Jay Lan wrote:
Andrew Morton wrote:
But the overhead at present is awfully low. If we don't need this
ability
at present (and I don't think we do) then a paper design would be
sufficient at this time. As long as we know we can do this in the
future
without breaking existing APIs then OK.
i can see if an exiting process is the only process in the thread group,
the (not is_thread_group) condition would be true. So, that leaves
multi-threaded applications that are not interested in tgid-data still
receive 2x taskstats data.
Jay,
Why is the 2x taskstats data for the multithreaded app a real problem ?
When differnt clients agree to use a common taskstats structure, they
also incur the potential
overhead of receiving extra data they don't really care about (in CSA's
case, that would be all the
delay accounting fields of struct taskstats). Isn't that, in some sense,
the "price" of sharing a structure
or delivery mechanism ?
You are mixing the two types of overhead: 1) overhead due to tgid,
2) overhead due to extra fields of struct taskstats they don't care
about.
The type 2 overhead for CSA is very small, but is bigger for you. In our
discussion earlier, i told you (and you accpeted) that i will insert
128 bytes of data into taskstat struct. I have not finalized the CSA
work yet, but it can be 168 additional bytes or close to that number:
/* Common Accounting Fields start */
u32 ac_uid; /* User ID */
u32 ac_gid; /* Group ID */
u32 ac_pid; /* Process ID */
u32 ac_ppid; /* Parent process ID */
struct timespec start_time; /* Start time */
struct timespec exit_time; /* Exit time */
u64 ac_utime; /* User CPU time [usec] */
u64 ac_stime; /* SYstem CPU time [usec] */
/* Common Accounting Fields end */
/* CSA accounting fields start */
u64 ac_sbu; /* System billing units */
u16 csa_revision; /* CSA Revision */
u8 csa_type; /* Record types */
u8 csa_flag; /* Record flags */
u8 ac_stat; /* Exit status */
u8 ac_nice; /* Nice value */
u8 ac_sched; /* Scheduling discipline */
u8 pad0; /* Unused */
u64 acct_rss_mem1; /* accumulated rss usage */
u64 acct_vm_mem1; /* accumulated virtual memory
usage */
u64 hiwater_rss; /* High-watermark of RSS usage
*/
u64 hiwater_vm; /* High-water virtual memory
usage */
u64 ac_minflt; /* Minor Page Fault */
u64 ac_majflt; /* Major Page Fault */
u64 ac_chr; /* bytes read */
u64 ac_chw; /* bytes written */
u64 ac_scr; /* read syscalls */
u64 ac_scw; /* write syscalls */
u64 ac_jid; /* Job ID */
/* CSA accounting fields end */
This is type 2 overhead. The bigger overhead in type 2, the bigger
impact of sending tgid data is bigger.
Of course, if this overhead becomes too much, we need to find
alternatives. But, as already shown,
even in the extreme case where app does nothing but fork/exit, there is
very
little performance impact. So I don't see how in the common case of
multithreaded apps, where exits
are going to be at a far lesser rate, the extra per-tgid data is a real
issue.
Yes, application handles "real" work between fork and exit. But,
each task within a thread group still trigger do_exit on termination,
right?
So, are we trying to solve a real problem ?
I do not know, but i am concerned. I will run some testing with the
taskstats struct above and get some data.
Thanks,
- jay
I'll address the alternatives in a separate mail but lets address this
point first please.
--Shailabh
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