TYPO3 Core

Please check your system status with

ipcs -s

sysctl -a |grep kernel\.sem

The default on Ubuntu 14.04 is:

kernel.sem = 32000      1024000000      500     32000

I get this:

root@matw:~# ipcs -s

------ Semaphore Arrays --------
key        semid      owner      perms      nsems     
0x0baa3533 0          www-data   666        3         
0x7c9e0990 32769      www-data   666        3         
0xa8e44899 65538      www-data   666        3         
0x1e536c5e 98307      www-data   666        3         
0xb4375310 131076     www-data   666        3         
0xea7be8dc 163845     www-data   666        3         
0x456c2c52 196614     www-data   666        3         
0x02867477 229383     www-data   666        3         
0xf0e544b5 262152     www-data   666        3         
0x339a6321 294921     www-data   666        3         
0x3ca1cf4d 327690     www-data   666        3         
0x4d9d2850 360459     www-data   666        3         
0xb532d7b8 393228     www-data   666        3         
0x5d85f034 425997     www-data   666        3         
0xd2358e97 458766     www-data   666        3         
0xaf36479e 491535     www-data   666        3         
0xad937213 524304     www-data   666        3         
0x2347b1c0 557073     www-data   666        3         
0x14362d4d 589842     www-data   666        3         
0xb1063a0a 622611     www-data   666        3         
0xa0784163 655380     www-data   666        3         
0xa1cc990c 688149     www-data   666        3         
0x87e611ee 720918     www-data   666        3         
0xc2b3fab1 753687     www-data   666        3         
0x33e81829 786456     www-data   666        3         
0xd8c99d51 819225     www-data   666        3         
0x8783b42a 851994     www-data   666        3         
0x60e5873a 884763     www-data   666        3         
0xc314ead5 917532     www-data   666        3         
0xa9442db3 950301     www-data   666        3         
0xab52b406 983070     www-data   666        3         
0x26551e77 1015839    www-data   666        3         
0xd3fc66ce 1048608    www-data   666        3         
0xf16f6100 1081377    www-data   666        3         
0x25b03396 1114146    www-data   666        3         
0x2ba70226 1146915    www-data   666        3         
0xf10f5538 1179684    www-data   666        3         
0x365b4f31 1212453    www-data   666        3         
0x6a8d5d1a 1245222    www-data   666        3         
0x6e2a442c 1277991    www-data   666        3         
0x8264f29c 1310760    www-data   666        3         
0x0973ce0a 1343529    www-data   666        3         
0x043b027a 1376298    www-data   666        3         
0xa250f4df 1409067    www-data   666        3         
0x7c984307 1441836    www-data   666        3         
0xebf7e556 1474605    www-data   666        3         
0xf02169f2 1507374    www-data   666        3         
0xc27603cd 1540143    www-data   666        3         
0xd3c3c612 1572912    www-data   666        3         
0x9cc25ec5 1605681    www-data   666        3         
0x04629384 1638450    www-data   666        3         
0xd5e85eaf 1671219    www-data   666        3         
0x22687898 1703988    www-data   666        3         
0xb40f30b2 1736757    www-data   666        3         
0xe31f7de7 1769526    www-data   666        3         
0x07aecb8a 1802295    www-data   666        3         
0xcf62bda7 1835064    www-data   666        3         
0x2c784309 1867833    www-data   666        3         
0x0e8d31f2 1900602    www-data   666        3         
0x4fd013e4 1933371    www-data   666        3         
0xb44b91b2 1966140    www-data   666        3         
0x9b85d8b5 1998909    www-data   666        3         
0xe8994ff1 2031678    www-data   666        3         
0xdeb8161d 2064447    www-data   666        3         
0xa3c4fb43 2097216    www-data   666        3         
0x2ba5a06e 2129985    www-data   666        3         
0xd33d736a 2162754    www-data   666        3         
0x113edfef 2195523    www-data   666        3         
0x36a1dc1a 2228292    www-data   666        3         
0xf12befbb 2261061    www-data   666        3         
0x69911364 2293830    www-data   666        3         
0xa9140a5f 2326599    www-data   666        3         
0x03c74337 2359368    www-data   666        3         
0x6a014e24 2392137    www-data   666        3         
0xbc24ca33 2424906    www-data   666        3         
0xf114402a 2457675    www-data   666        3         
0x2a36aca6 2490444    www-data   666        3         
0x2e918a24 2523213    www-data   666        3         
0xe4a3c031 2555982    www-data   666        3         
0x0c841d24 2588751    www-data   666        3         
0xb380209d 2621520    www-data   666        3         
0x6149871e 2654289    www-data   666        3         

root@matw:~# sysctl -a |grep kernel\.sem
kernel.sem = 250    32000    32    128
kernel.sem_next_id = -1

So this is my system error? Not TYPO3?

What server do you use?
Is this a shared hosting or so?

You have a 250 limit for the number of semaphores.

TYPO3 currently does not remove them, because we have no "controlled" environment - in the sense that a request does not know about other requests - and we don't want to have race conditions because of removing a semaphore too early.

Thanks for you report, btw, because this is very valuable feedback as the Locking API was changed not long ago and we really need field experience.

Currently the semaphore locking is the preferred method, if available. But if it turns out that it causes too much trouble, we might change that again.

Thanks, for this information. So what can I do when this error appears again?
This site is on DigitalOcean with the smallest virtual server (512MB ram, 1 core, 20 GB SSD).

If you want to check it you can use this link to register and get 10$ for free https://www.digitalocean.com/?refcode=96665686914b

More infos about the numbers.

kernel.sem = 32000      1024000000      500     32000
kernel.sem = 250        32000           32      128

The order is: SEMMSL, SEMMNS, SEMOPM, and SEMMNI

SEMMSL: maximum number of semaphores per semaphore set
SEMMNS: total number of semaphores (not semaphore sets) for the entire Linux system
SEMOPM: maximum number of semaphore operations that can be performed per semop(2) system call
SEMMNI: maximum number of semaphore sets for the entire Linux system

Description of the settings eg. http://www.puschitz.com/TuningLinuxForOracle.shtml#SettingSemaphores

It can clearly be seen that the second set of numbers limits the total number of semaphores on the whole system to 128(!)

So what can I do when this error appears again?

Search for how you can increase the maximum number of semaphores or how you can release existing ones on the web.

After digging around in the code I realize:

We use a lot of information to generate a unique key for the lock. So we actually have a key per variation of a page. (md5 of serialize of id, type,gr_list, MP, cHash, startPage)
\TYPO3\CMS\Frontend\Controller\TypoScriptFrontendController::createHashBase(TRUE)

This means we spam the system with a semaphore per variation, but OS might limit us to 128 semaphores!
This approach is brutally wrong, even if we don't use semaphores but file locks, which need a file per variation then.

We need to define a better strategy.
Idea: Use a single lock to access some shared resource, which keeps track of the process currently rendering a page.

Current procedure:

• check cache, if empty proceed with
• generate hash
• get lock for this hash
• generate page
• write page to cache
• release lock

This creates a lock per hash, which might be manifold.

I propose the following procedure:

• check cache, if empty proceed with
• generate hash
• get lock for cache access (1 unique lock per instance)
• write to cache that we're working on the content (set lock for this hash)
• release the lock
• generate page
• get lock again
• remove lock for the hash
• write page to cache
• release lock

This is some sort of simple reader/writer pattern, where we allow inconsistent reads, while generating the page.

Regarding the Semphore locking in general:
We need to change the semaphore keys. Important to say that the keys must be unique per instance, otherwise two T3 instances on the same server might block each other (even if only for a very short time).
The best way to generate a key (that also does not collide with OS keys) is to use ftok(filename, projId), which needs a filename.
We could use a fixed filename, but that would limit the possible range of projId to 256 (8bit), which is a no-go, since we can't map arbitrary $subject to 256 projIds in a sane way.
Therefore we propose to create a file in typo3temp/locks/sem_<md5 of $subject> used for ftok.
This way one also has a reference how many semaphores have been created in the system by the instance, without looking into command line tools.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/38840

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/38840

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/38840

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/38840

Patch set 6 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/38840

Patch set 7 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/38840

Testing instructions:

Insert a sleep(10) in the tsfe::generate_pre...() function, right after the release of the lock. This allows you to see the "Page is generated" message" if you visit the page with two browsers at the same time. Clear the cache first and logout from BE.
At the same time you can watch the lock files come and go in typo3temp/locks.

Patch set 8 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/38840

Patch set 9 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/38840

Patch set 10 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/38840

Test-script to produce some locking states, this would result in deadlock with patchset 8 because of missing no block option of patchset 9

#!/bin/sh
./typo3cms cache:flushgroups --groups=pages
time wget --output-document=index_temp.1.html --content-on-error "http://localhost/" &
time wget --output-document=index_temp.2.html --content-on-error "http://localhost/" &
time wget --output-document=index_temp.3.html --content-on-error "http://localhost/" &
time wget --output-document=index_temp.4.html --content-on-error "http://localhost/" &
time wget --output-document=index_temp.5.html --content-on-error "http://localhost/" &
time wget --output-document=index_temp.6.html --content-on-error "http://localhost/" &
time wget --output-document=index_temp.7.html --content-on-error "http://localhost/" &
time wget --output-document=index_temp.8.html --content-on-error "http://localhost/" &
time wget --output-document=index_temp.9.html --content-on-error "http://localhost/" &
time wget --output-document=index_temp.10.html --content-on-error "http://localhost/" &
time wget --output-document=index_temp.11.html --content-on-error "http://localhost/" &
time wget --output-document=index_temp.12.html --content-on-error "http://localhost/" &

Regarding usleep in locking part it according to times it seems to be better to increase it (at least for me). But I think it is also depends on how much memory/resources are available
With 5us I am about 1second slower per request than with e.g. 5000 but I guess that really depends on the system usage.
But I think 5us might really be to little 5000 would at least be 5 milliseconds.
Event better (but not much normally than 5000) seem to be 50000 or 100000 but might also be system related.

Maybe others can do some tests too?

Patch set 11 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/38840

Patch set 12 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at http://review.typo3.org/38840