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SubjectRe: async I/O seems to be blocking on 2.6.15
On Fri, 3 Nov 2006, Jens Axboe wrote:

> On Fri, Nov 03 2006, Brent Baccala wrote:
>>
>> ...
>>
>> The enqueues still take a noticable amount of time, though, just a lot
>> less than before. They average 1 second total. That's 100 one-MB
>> reads, broken down into 128 KB blocks, I suppose, for a total of 800
>> low-level reads. Setting nr_requests higher (2048) doesn't seem to do
>> any more good.
>>
>> I can see that you've put me on the right track, but I am still
>> puzzling... any idea what the remaining second is being used for?
>
> Try to time it (visual output of the app is not very telling, and it's
> buffered) and then apply some profiling.
>

Yeah, I did time it. I bracketed the enqueues with calls to
gettimeofday(). It takes them about a second to run, and I made sure
that the fprintfs were outside the timing checks. Here's the actual code:


fprintf(stderr, "Enqueues starting\n");
gettimeofday(&tv1, NULL);

for (i=0; i<NUMAIOS; i++) {

memset(&iocb[i], 0, sizeof(struct iocb));

iocb[i].aio_lio_opcode = IOCB_CMD_PREAD;
iocb[i].aio_fildes = fd;
iocb[i].aio_buf = (unsigned long) buffer[i];
iocb[i].aio_nbytes = BUFFER_BYTES;
iocb[i].aio_offset = BUFFER_BYTES * i;
/* aiocb[i].aio_offset = 0; */

iocbp[0] = &iocb[i];
if (io_submit(aio_default_context, 1, iocbp) != 1) {
perror("");
fprintf(stderr, "Can't enqueue aio_read %d\n", i);
}
}

gettimeofday(&tv2, NULL);
subtract_timeval(&tv2, &tv1);
sprint_timeval(strbuf, &tv2);
fprintf(stderr, "Enqueues complete in %s\n", strbuf);


And the output:


baccala@debian ~/src/endgame$ time ./testaio
Enqueues starting
Enqueues complete in 1.187s

real 0m5.335s
user 0m0.001s
sys 0m0.760s
baccala@debian ~/src/endgame$


What kind of profiling did you have in mind? Kernel profiling? As
you can see, its user time is basically nil.



-bwb

Brent Baccala
cosine@freesoft.org
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