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From "Paul Davis" <paul.joseph.da...@gmail.com>
Subject Re: Closing Files
Date Sun, 23 Nov 2008 19:54:17 GMT
After waking up and poking around trunk with a few log statements I
managed to unravel a clearer picture of what's going on.

Near as I can tell the update_loops have a 10 second timeout in
get_notify_pids and when this fails it cascades and ends up closing
the file handles.

Instead of first trying to directly mirror that paradigm i attempted
to use the reference counting semantics available in couch_file to
have files automatically closed when a request ends by upping the ref
count when a process registers to use an update group. This almost
works.

Doing a couple tests it looks like mochiweb is using a single process
for an entire Keep-Alive connection. Thus when under load from the
Futon tests, it can still run out of descriptors. Testing from curl
has the expected effect of closing the FD for every request.

The next two ideas I had were to use either an LRU cache similar to
couch_server.erl or add a try/after block around requests in
couch_httpd_view.erl. I'm starting to lean towards the slight changes
to couch_httpd_view.erl at the moment because it seems to be the
better of the two even though I was trying to keep changes to it as
minimal as possible. (I've changed about 5-10 lines so far, and those
were mostly changing module names on function calls.)

Anyone got any input?




On Sun, Nov 23, 2008 at 3:12 AM, Paul Davis <paul.joseph.davis@gmail.com> wrote:
> Totally forgot to paste the link after I explicitly pushed code.
>
> http://github.com/davisp/couchdb/tree/update-false
>
> Purge works fine for me.
>
> Some things to note about the code:
>
> I finally made a couch_os_process that can be used for the cases where
> we have external processes like the action2 external stuff.
>
> The overall restructuring looks like this:
>
> 1. couch_view sets up couch_view_group gen_servers that perform the
> actual update.
> 2. couch_view_group 'owns' a single view_updater process that
> basically iterates over enum_docs_since_seq and throws them at
> couch_query_server
> 3. couch_query_server cast's incoming docs to N couch_os_processes
> round robin style.
> 4. Using a closure, the couch_os_process's send the key/value output
> back to couch_view_group which buffers and flushes to disk.
> 5. When couch_view_group recieves as many results as view_updater sent
> out, it terminates sending the final group to any process that
> registered to get a copy of it.
>
> Also, at the moment I think this implementation might be slower than
> the original, but I have a sneaking suspicion that I'm calling fsync
> quite a few times that I shouldn't be.
>
> update-false isn't actually in yet, but it'll fit nicely into
> couch_view:get_map_view with a call to get the current version of the
> Group record stored in the gen_server state.
>
> Also, I realized it would be fairly trivial to add a wait=N to wait at
> most N seconds in case people think that's a good idea. I'm on the
> fence.
>
> And it's time for bed before my brain explodes.
>
> On Sun, Nov 23, 2008 at 2:51 AM, Chris Anderson <jchris@apache.org> wrote:
>> On Sat, Nov 22, 2008 at 11:44 PM, Paul Davis
>> <paul.joseph.davis@gmail.com> wrote:
>>> I start getting EMFILE errors indicating i've
>>> exhausted file handles. I cannot for the life of me piece together how
>>> file handles are cleaned up *after* the view has been completely
>>> written to the client.
>>>
>>
>> I had the same error, but it would show up when attempting to view
>> after a purge (which would never return to the client), so I bet it
>> was a different cause. (Mine kept spawning new view updates, over and
>> over, due to an off-by-one error when the seq-id decremented)
>>
>> Is your version of code up on a github branch?
>>
>> Chris
>>
>>
>> --
>> Chris Anderson
>> http://jchris.mfdz.com
>>
>

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