On 2012-02-08 10.34, Jan Stary wrote:
> On Feb 08 08:25:49, Stuart Henderson wrote:
>> On 2012-02-07, Joe Gidi <j...@entropicblur.com> wrote:
>>> In every case, when the box hangs, I'm unable to break into ddb.
>>
>> How long do you leave it when it hangs? There have been occasions
>> where a box appears to hang but then recovers.
>>
>> Are you using softdep?
>
> This interests me. The OP's problem was what appeared
> to be a hang during "make -j8 build'. I have experienced
> something similar while doing heavy IO operations.
>
> If I cvs up many repositories at once, while
> dump|restoring a few filesystems at once,
> the box *sometimes* seems totaly unresponsive,
> only to react to my keybord stroke _a_long_time_
> later; sometimes, it is unresponsive locally,
> but can be ssh'd to - but the login process doesn't
> make it to actually spawning a shell.
>
> Yes, I am usign softdep, almost everywhere.
>
> Stuart, could you please elaborate on how this (possibly) happens,
> and what is the role of softdep in it?
I've seen this too, under similar circumstances.
I haven't investigated it further, but I suspect that when the file
system in softdep mode needs to write out a whole bunch of metadata
at once (which I believe it does in 30s intervals, if nothing else
(read: lots of metadata-altering activity) makes the buffers fill up
prematurely) it does so in one go, without releasing locks and/or
enabling interrupts as it works its way through committing dirty
buffers to disk.
Whatever the underlying cause, it's of course not a desired effect,
since it suspends virtually any other activity in the box, whether
disk related or not for a loooong time.
A work-around for this is probably to not use softdep, but this moves
the performance penalty elsewhere which may or may not be acceptable
to a specific use case.
I think in the long run OpenBSD:s i/o scheduling and file system
options might need an overhaul, but that's a different discussion.
(For example, I'd love to see Jeff Robertson's and Kirk McKusick's
work on soft update journaling that went into FreeBSD 9 in OpenBSD
as well. Had I the time I'd look into it myself (it's a *lot* of work
from what little I've seen of it, but no doubt it would be FUN work)
but alas I don't at the moment, so all I can do is post this wish. :-)
Regards,
/Benny
--
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