Re: ioWait brown out

Are they created one at a time, or are you creating and writing to all
of them simultaneously?  If the latter, it's possible that the seek
times are what are killing you.  Are you doing anything that would
force writes (like fsync) and prevent the OS caching from working?
What kind of write rates do you see writing to a single file using dd?

Matthew Bowie
Programmer/IT Consultant
[email protected]



On Fri, Feb 4, 2011 at 10:33 AM, Aaron Birenboim <[email protected]> wrote:
> I have a program which is trying to write several dozen
> 0.25 to 8 MByte files/sec, when recording diagnostics.
>
> My machine gets into a high ioWait situation, and "browns-out".
> The machine barely runs for several minutes.
>
> The test code can complete, and the high ioWait continues
> for several minutes.
>
> After the program is done, I can do "sudo sync",
> which will take several minutes to complete,
> then the machine is OK.
>
> I have repeated this on several machines, same results.
> I haven even tried writing to NFS... the client machine
> browns-out, the server is fine.
>
> I am not talking about some amazingly massive amount of data here.
> If things worked, I would be recording about 1G in about 2 sec.
> I realize that I should only be able to expect (at worst)
> about 50MBytes/sec to disk, hence my 2-second run (no diagnostic
> file writes) should take about 20 sec with the diagnostic data dump.
> However, this run takes more like 10 minutes, and my machine
> goes wanky.
>
> Any idea what might do this?
>
> In general, I am new'ing a 1-2 MB buffer, filling it with
> data (2D arrays of floats), then writing it to a file,
> then delete[] 'ing the buffer ...
> as fast as I can, for some 300 files or so.
>
> Is it not reasonable for me to be able to
> create these mid-sized files with a bandwidth
> of around (at least) 50MBytes/sec to SATA-II, running ext4?
>
> =A0 =A0 =A0 =A0 =A0 =A0aaron
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