Nginx nginx: [emerg] could not build map_hash, you should increase map_hash_bucket_size

negative · Dec 10, 2015

Hello

I have created a custom rewrite list including 500 url to new urls manually. And i set the map module in the nginx.conf. But when i include these URLS, nginx cant restart

Nginx gives error (via journalctl -xn) that:

Dec 10 01:35:22 server nginx[27216]: Starting nginx: nginx: [emerg] could not build map_hash, you should increase map_hash_bucket_size: 64
Dec 10 01:35:22 server nginx[27216]: [FAILED]
Dec 10 01:35:22 server systemd[1]: nginx.service: control process exited, code=exited status=1
Click to expand...

I didn't find map_hash_bucket_size in the nginx.conf but i tried to add these value. But if i put this variable to nginx.conf, nginx gives error again what about "duplicate entry bucket_size" So where store this value i didnt understand ?

Also, i tried that if i run 60 URL's for rewrite, nginx restarting and runs without problem. But if i enter more than 64, i need increase this. So, if i have 500 URL, must i change this value to 512 ? And how ?

Thanks.

eva2000 · Dec 10, 2015

see Module ngx_http_map_module

Sets the bucket size for the map variables hash tables. Default value depends on the processor’s cache line size. The details of setting up hash tables are provided in a separate document.
Click to expand...

as to cache line size see and install hwlock hwloc and lstopo | Centmin Mod Community

or
Code:
cat /proc/cpuinfo
not sure if it's related to
Code:
cache_alignment : 64
try in nginx.conf within http {} server context
Code:
map_hash_bucket_size 128;
or
Code:
map_hash_bucket_size 192;
or
Code:
map_hash_bucket_size 256;
and Setting up hashes

To quickly process static sets of data such as server names, map directive’s values, MIME types, names of request header strings, nginx uses hash tables. During the start and each re-configuration nginx selects the minimum possible sizes of hash tables such that the bucket size that stores keys with identical hash values does not exceed the configured parameter (hash bucket size). The size of a table is expressed in buckets. The adjustment is continued until the table size exceeds the hash max size parameter. Most hashes have the corresponding directives that allow changing these parameters, for example, for the server names hash they are server_names_hash_max_size and server_names_hash_bucket_size.

The hash bucket size parameter is aligned to the size that is a multiple of the processor’s cache line size. This speeds up key search in a hash on modern processors by reducing the number of memory accesses. If hash bucket size is equal to one processor’s cache line size then the number of memory accesses during the key search will be two in the worst case — first to compute the bucket address, and second during the key search inside the bucket. Therefore, if nginx emits the message requesting to increase either hash max size or hash bucket size then the first parameter should first be increased.
Click to expand...

negative · Dec 10, 2015

Hello

I put the map_hash_bucket_size 128; to top of the http{} section in nginx.conf and problem is fixed. Nginx has restarted without problem and rewrite rules working like a charm. Thanks ...

eva2000 · Dec 10, 2015

Glad to hear

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Nginx nginx: [emerg] could not build map_hash, you should increase map_hash_bucket_size

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Nginx nginx: [emerg] could not build map_hash, you should increase map_hash_bucket_size

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