linux-kernel - Re: async I/O seems to be blocking on 2.6.15

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Date:	Fri, 3 Nov 2006 12:09:50 -0500 (EST)
From:	Brent Baccala <cosine@...esoft.org>
To:	Jens Axboe <jens.axboe@...cle.com>
cc:	linux-kernel@...r.kernel.org
Subject: Re: 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@...ian ~/src/endgame$ time ./testaio
Enqueues starting
Enqueues complete in 1.187s

real    0m5.335s
user    0m0.001s
sys     0m0.760s
baccala@...ian ~/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@...esoft.org
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