If an admin finds it necessary to configure
url_rewrite_children 16 concurrency=4
the helper subsystem is theoretically capable of processing 64 messages simultaneously.
It does not makes sens to use max(2*4,2*16)=32 for queue-size, but should be _at least_ 64.
Since Squid (before introducing the concurrency parameter) wanted to have a queue that is twice the capacity of the helper subsystem (most likely since this was found to be a good value), the logical
thing to do is what I proposed and use 2*NCONC*NCHILD, which in the above example is 2*4*16 = 128.
There are many messages on this mailing list about problems with helpers.
One refers to
external_acl_type session concurrency=100 ttl=3 negative_ttl=0 children-max=1 %LOGIN /usr/lib64/squid/ext_session_acl -a -T 60 -b /var/lib/squid/sessions/
with NCONC=100, NCHILD=1, it is not hard to imagine that a default queue-size of 2 is problematic.
Another one has
store_id_children 100 startup=0 idle=1 concurrency=1000
with NCONC=1000 and NCHILD=100 the admin wants to process max 1 million requests simultaneously.
*I have a suspicion that admins use these large numbers in an attempt to have a good performance but they never reach it since the queue-size stays small.*
So, the impact of queue-size should be very clearly documented, better is to use a sane default.
Marcus
On 03/07/18 12:07, Amish wrote:
2*NCONC*NCHILD will possibly lead to too high value as a default and the busy-ness will never be logged.
My proposal of higher of (2*NCONC) and (2*NCHILD) would mean that load is now regularly high enough that atleast 2 more children are needed.
We can start with that and then find a better formula.
Amish
On Tuesday 03 July 2018 07:49 PM, Marcus Kool wrote:
The original intention of this default value is have a queue that is twice the size of the messages being processed, so for helpers with concurrency=NCONC and num_children=NCHILD it makes a lot of
sense to set the default queue length to 2*NCONC*NCHILD.
I do not understand that "compatibility" with doing the wrong calculation is a good thing.
Marcus
On 03/07/18 05:16, Amos Jeffries wrote:
On 03/07/18 20:00, Amish wrote:
Hello,
In squid 4.1 new option "queue-size" was introduced.
In most (or all) cases default "queue-size" is set to children-max*2.
But I believe it should be higher of (children-max*2) OR (concurrency*2)
Or it can be some better formula but the point I am trying to make is
that, "concurrency" should be taken in to account for calculating
default value of "queue-size".
Please consider.
FYI; When we add a directive or option to control some behaviour that
already happens the default is usually set to the value all existing
Squid are using so nobody gets an unexpected surprise with upgrade.
That can change later once more people have experimented to find what a
better value actually is.
Cheers
Amos
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