[Haskell-cafe] Timeout on pure code

Gregory Collins greg at gregorycollins.net
Wed Apr 22 20:09:33 UTC 2015

Maybe but it would be helpful to rule the scenario out. Johan's ekg library
is also useful, it exports a webserver on a different port that you can use
to track metrics like gc times, etc.

Other options for further debugging include gathering strace logs from the
binary. You'll have to do some data gathering to narrow down the cause
unfortunately -- http client? your code? Snap server? GHC event manager
(System.timeout is implemented here)? GC? etc


On Wed, Apr 22, 2015 at 10:14 AM, Kostiantyn Rybnikov <k-bx at k-bx.com> wrote:

> Gregory,
> Servers are far from being highly-overloaded, since they're currently
> under a much less load they used to be. Memory consumption is stable and
> low, and there's a lot of free RAM also.
> Would you say that given these factors this scenario is unlikely?
> On Wed, Apr 22, 2015 at 7:56 PM, Gregory Collins <greg at gregorycollins.net>
> wrote:
>> Given your gist, the timeout on your requests is set to a half-second so
>> it's conceivable that a highly-loaded server might have GC pause times
>> approaching that long. Smells to me like a classic Haskell memory leak
>> (that's why the problem occurs after the server has been up for a while):
>> run your program with the heap profiler, and audit any shared
>> tables/IORefs/MVars to make sure you are not building up thunks there.
>> Greg
>> On Wed, Apr 22, 2015 at 9:14 AM, Kostiantyn Rybnikov <k-bx at k-bx.com>
>> wrote:
>>> Hi!
>>> Our company's main commercial product is a Snap-based web app which we
>>> compile with GHC 7.8.4. It works on four app-servers currently
>>> load-balanced behind Haproxy.
>>> I recently implemented a new piece of functionality, which led to weird
>>> behavior which I have no idea how to debug, so I'm asking here for help and
>>> ideas!
>>> The new functionality is this: on specific url-handler, we need to query
>>> n external services concurrently with a timeout, gather and render results.
>>> Easy (in Haskell)!
>>> The implementation looks, as you might imagine, something like this
>>> (sorry for almost-real-haskell, I'm sure I forgot tons of imports and other
>>> things, but I hope everything is clear as-is, if not -- I'll be glad to
>>> update gist to make things more specific):
>>> https://gist.github.com/k-bx/0cf7035aaf1ad6306e76
>>> Now, this works wonderful for some time, and in logs I can see both,
>>> successful fetches of external-content, and also lots of timeouts from our
>>> external providers. Life is good.
>>> But! After several days of work (sometimes a day, sometimes couple
>>> days), apps on all 4 servers go crazy. It might take some interval (like 20
>>> minutes) before they're all crazy, so it's not super-synchronous. Now: how
>>> crazy, exactly?
>>> First of all, this endpoint timeouts. Haproxy requests for a response,
>>> and response times out, so they "hang".
>>> Secondly, logs are interesting. If you look at the code from gist once
>>> again, you can see, that some of CandidateProvider's don't actually require
>>> any networking work, so all they do is actually just logging that they're
>>> working (I added this as part of debugging actually) and return pure data.
>>> So what's weird is that they timeout also! Here's how output of our logs
>>> starts to look like after the bug happens:
>>> ```
>>> [2015-04-22 09:56:20] provider: CandidateProvider1
>>> [2015-04-22 09:56:20] provider: CandidateProvider2
>>> [2015-04-22 09:56:21] Got timeout while requesting CandidateProvider1
>>> [2015-04-22 09:56:21] Got timeout while requesting CandidateProvider2
>>> [2015-04-22 09:56:22] provider: CandidateProvider1
>>> [2015-04-22 09:56:22] provider: CandidateProvider2
>>> [2015-04-22 09:56:23] Got timeout while requesting CandidateProvider1
>>> [2015-04-22 09:56:23] Got timeout while requesting CandidateProvider2
>>> ... and so on
>>> ```
>>> What's also weird is that, even after timeout is logged, the string
>>> ""Got responses!" never gets logged also! So hanging happens somewhere
>>> in-between.
>>> I have to say I'm sorry that I don't have strace output now, I'll have
>>> to wait until this situation happens once again, but I'll get later to you
>>> with this info.
>>> So, how is this possible that almost-pure code gets timed-out? And why
>>> does it hang afterwards?
>>> CPU and other resource usage is quite low, number of open
>>> file-descriptors also (it seems).
>>> Thanks for all the suggestions in advance!
>>> _______________________________________________
>>> Haskell-Cafe mailing list
>>> Haskell-Cafe at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>> --
>> Gregory Collins <greg at gregorycollins.net>

Gregory Collins <greg at gregorycollins.net>
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