[squeak-dev] Server timeouts and 504 return codes

John Pfersich smalltalker2 at mac.com
Mon Jan 28 02:56:38 UTC 2019


I know that it mostly a cost and reconfiguration thing, but has there been any thought to maybe make multiple servers? With the front end doing a round robin to distribute the load? I’m saying this without knowing what kind of loads the server is experiencing, or whether there are log files that record the activity. 

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> On Jan 27, 2019, at 18:45, Eliot Miranda <eliot.miranda at gmail.com> wrote:
> 
> Hi Levente,
> 
>>> On Jan 27, 2019, at 5:40 PM, Levente Uzonyi <leves at caesar.elte.hu> wrote:
>>> 
>>> On Sun, 27 Jan 2019, Chris Muller wrote:
>>> 
>>> Hi,
>>> 
>>>>>>> Yes, the SqueakMap server image is one part of the dynamic, but I
>>>>>>> think another is a bug in the trunk image.  I think the reason Tim is
>>>>>>> not seeing 45 seconds before error is because the timeout setting of
>>>>>>> the high-up client is not being passed all the way down to the
>>>>>>> lowest-level layers -- e.g., from HTTPSocket --> WebClient -->
>>>>>>> SocketStream --> Socket.  By the time it gets down to Socket which
>>>>>>> does the actual work, it's operating on its own 30 second timeout.
>>>>>> 
>>>>>> I would expect subsecond reponse times. 30 seconds is just unacceptably
>>>>>> long.
>>>>> 
>>>>> Well, it depends on if, for example, you're in the middle of
>>>>> Antarctica with a slow internet connection in an office with a fast
>>>>> connection.  A 30 second timeout is just the maximum amount of time
>>>>> the client will wait for the entire process before presenting a
>>>>> debugger, that's all it can do.
>>>> 
>>>> We can be sure that Tim should get subsecond response times instead of
>>>> timeouts after 30 seconds.
>>> 
>>> Right, but timeout settings are a necessary tool sometimes, my point
>>> was that we should fix client code in trunk to make timeouts work
>>> properly.
>>> 
>>> Incidentally, 99% of SqueakMap requests ARE subsecond -- just go to
>>> map.squeak.org and click around and see.  For the remaining 1% that
>>> aren't, the issue is known and we're working on a new server to fix
>>> that.
>> 
>> Great! That was my point: the image needs to be fixed.
>> 
>>> 
>>>>>>> It is a fixed amount of time, I *think* still between 30 and 45
>>>>>>> seconds, that it takes the SqueakMap server to save its model after an
>>> 
>>> and so if in the meantime it can simply be made to wait 45s instead of
>>> 30s, then current SqueakMap will only be that occasional delay at
>>> worst, instead of the annoying debugger we currently get.
>> 
>> I don't see why that would make a difference: the user would get a debugger anyway, but only 15 seconds later.
>> 
>>> 
>>>>>> You would save seconds, not milliseconds by not downloading files again.
>>>>> 
>>>>> IIUC, you're saying we would save one hope in the "download" --
>>>>> instead of client <--> alan <--> andreas, it would just be client <-->
>>>>> alan.  Is that right?
>>>> 
>>>> No. If the client doesn't have the mcz in the package cache but nginx has
>>>> it in its cache, then we save the transfer of data between alan and
>>>> andreas.
>>> 
>>> Are alan and andreas co-located?
>> 
>> They are cloud servers in the same data center.
>> 
>>> 
>>>> The file doesn't have to be read from the disk either.
>>> 
>>> I assume you mean "read from disk" on alan?  What about after it's
>>> cached so many mcz's in RAM that its paging out to swap file?  To me,
>>> wasing precious RAM (of any server) to cache old MCZ file contents
>>> that no one will ever download (because they become old very quickly)
>>> feels wasteful.  Dragster cars are wasteful too, but yes, they are
>>> "faster"... on a dragstrip.  :)  I guess there'd have to be some kind
>>> of application-specific smart management of the cache...
>> 
>> Nginx's proxy_cache can handle that all automatically. Also, we don't need a large cache. A small, memory-only cache would do it.
>> 
>>> 
>>> Levente, what about the trunk directory listing, can it cache that?
>> 
>> Sure.
>> 
>>> That is the _#1 thing_ source.squeak.org is accessing and sending back
>>> over, and over, and over again -- every time that MC progress box that
>>> says, "Updating [repository name]".
>> 
>> Right, unless you update an older image.
>> 
>>> 
>>>> If the client does have the mcz, then we save the complete file transfer.
>>>> 
>>>>> 
>>>>> I don't know what the speed between alan <---> andreas is, but I doubt
>>>>> it's much slower than client <---> alan in most cases, so the savings
>>>>> would seem to be minimal..?
>>>> 
>>>> The image wouldn't have to open a file, read its content from the disk and
>>>> send that through a socket.
>>> 
>>> By "the image" I assume you mean the SqueakSource server image.  But
>>> opening the file takes very little time.  Original web-sites were
>>> .html files, remember how fast those were?  Plus, filesystems "cache"
>>> file contents into their own internal caches anyway...
>> 
>> Each file uses one external semaphore, each socket uses three. If you use a default image, there can be no more than 256 external semaphores which is ridiculous for a server, and it'll just grind to a halt when some load arrives. Every time the external semaphore table is full, a GC is triggered to try clear it up via the finalization process.
>> Reading a file into memory is slow, writing it to a socket is slow.
>> (Compared to nginx which uses sendfile to let the kernel handle that).
>> And Squeak can only use a single process to handle everything.
> 
> That’s configurable.  Alas because writing lock-free table growth is not easy the external semaphore table doesn’t grow automatically.  But the vm does allow its size to be specified in a value cached in the image header and read at startup (IIRC).  So we could easily have a 4K entry external semaphore table.
> 
>> 
>>> 
>>> Yes, it still has to return back through alan but I assume alan does
>>> not wait for a "full download" received from andreas before its
>>> already pipeing back to the Squeak client.  If true, then it seems
>>> like it only amounts to saving one hop, which would hardly be
>>> noticeable over what we have now.
>> 
>> The goal of caching is not about saving a hop, but to avoid handling files in Squeak.
>> 
>>> 
>>>> Nginx does that thing magnitudes faster than
>>>> Squeak.
>>> 
>>> The UX would not be magnitudes faster though, right?
>> 
>> Directly by letting nginx serving the file, no, but the server image would be less likely to get stalled (return 5xx responses).
>> But the caching scheme I described in this thread would make the UX a lot quicker too, because data would not have to be transferred when you already have it.
>> 
>>> 
>>>>>>>> That would also let us save bandwidth by not downloading files already
>>>>>>>> sitting in the client's package cache.
>>>>>>> 
>>>>>>> How so?  Isn't the package-cache checked before hitting the server at
>>>>>>> all?  It certainly should be.
>>>>>> 
>>>>>> No, it's not. Currently that's not possible, because different files can
>>>>>> have the same name. And currently we have no way to tell them apart.
>>>>> 
>>>>> No.  No two MCZ's may have the same name, certainly not withiin the
>>>>> same repository, because MCRepository cannot support that.  So maybe
>>>> 
>>>> Not at the same time, but it's possible, and it just happened recently
>>>> with Chronology-ul.21.
>>>> It is perfectly possible that a client has a version in its package cache
>>>> with the same name as a different version on the server.
>>> 
>>> But we don't want to restrict what's possible in our software design
>>> because of that.  That situation is already a headache anyway.  Same
>>> name theoretically can come only from the same person (if we ensure
>>> unique initials) and so this is avoidable / fixable by resaving one of
>>> them as a different name...
>> 
>> It wasn't me who created the duplicate. If your suggestion had been in place, some images out there, including mine, would have been broken by the update process.
>> 
>>> 
>>>>> we need project subdirectories under package-cache to properly
>>>>> simulate each cached Repository.  I had no idea we were neutering 90%
>>>>> of the benefits of our package-cache because of this too, and just
>>>>> sitting here, I can't help wonder whether this is why MCProxy doesn't
>>>>> work properly either!
>>>>> 
>>>>> The primary purpose of a cache is to *check it first* to speed up
>>>>> access to something, right?  What you say about package-cache sounds
>>>> 
>>>> I don't know. It wasn't me who designed it. :)
>>> 
>>> I meant ANY "cache".
>>> 
>>> https://en.wikipedia.org/wiki/Cache_(computing)
>> 
>> It still depends on the purpose of the cache. It's possible that package-cache is just a misnomer or it was just a plan to use it as a cache which hasn't happened yet.
>> 
>>> 
>>> For Monticello, package-cache's other use-case is when an
>>> authentication issue occurs when trying to save to a HTTP repository.
>>> At that point the Version object with the new ancestry was already
>>> constructed in memory, so rather than worry about trying to "undo" all
>>> that, it was simpler and better to save it to a package-cache, persist
>>> it safely so the client can simply move forward from there (get access
>>> to the HTTP and copy it or whatever).
>> 
>> The package-cache is also handy as a default repository and as an offline storage.
>> 
>> Levente
>> 
>>> 
>>> - Chris
>>> 
>>>>> really bad we should fix that, not surrender to it.
>>>> 
>>>> Yes, that should be fixed, but it needs changes on the server side.
>>>> What I always had in mind was to extend the repository listing with
>>>> hashes/uuids so that the client could figure out if it needs to download a
>>>> specific version. But care must be taken not to break the code for
>>>> non-ss repositories (e.g. simple directory listings).
>>>> 
>>>> Levente
>>>> 
>>>>> 
>>>>> - Chris
>>>>> 
>>> 
>> 
> 
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