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Tuning options

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Advanced, development and debugging options

Hash Algorithm

Storage Backend

Storages can be used in vcl as storage.name, so, for example if myStorage was defined by -s myStorage=malloc,5G, it could be used in VCL like so:

set beresp.storage = storage.myStorage;

A special name is Transient which is the default storage for uncacheable objects as resulting from a pass, hit-for-miss or hit-for-pass.

If no -s options are given, the default is:

-s default,100m

If no Transient storage is defined, the default is an unbound default storage as if defined as:

-s Transient=default

The following storage types and options are available:

Jail

-j solaris,worker=basic

Management Interface

The commands available are documented in varnish-cli(7).

CLI Command File

vcl.load panic /etc/varnish_panic.vcl
vcl.load siteA0 /etc/varnish_siteA.vcl
vcl.load siteB0 /etc/varnish_siteB.vcl
vcl.load siteC0 /etc/varnish_siteC.vcl
vcl.label siteA siteA0
vcl.label siteB siteB0
vcl.label siteC siteC0
vcl.load main /etc/varnish_main.vcl
vcl.use main

Every line in the file, including the last line, must be terminated by a newline or carriage return.

If a command in the file is prefixed with '-', failure will not abort the startup.

Note that it is necessary to include an explicit vcl.use command to select which VCL should be the active VCL when relying on CLI Command File to load the configurations at startup.

Run Time Parameter Flags

Default Value Exceptions on 32 bit Systems

List of Parameters

accept_filter

Enable kernel accept-filters. This may require a kernel module to be loaded to have an effect when enabled.

Enabling accept_filter may prevent some requests to reach Varnish in the first place. Malformed requests may go unnoticed and not increase the client_req_400 counter. GET or HEAD requests with a body may be blocked altogether.

acceptor_sleep_decay

If we run out of resources, such as file descriptors or worker threads, the acceptor will sleep between accepts. This parameter (multiplicatively) reduce the sleep duration for each successful accept. (ie: 0.9 = reduce by 10%)

acceptor_sleep_incr

If we run out of resources, such as file descriptors or worker threads, the acceptor will sleep between accepts. This parameter control how much longer we sleep, each time we fail to accept a new connection.

acceptor_sleep_max

If we run out of resources, such as file descriptors or worker threads, the acceptor will sleep between accepts. This parameter limits how long it can sleep between attempts to accept new connections.

auto_restart

Automatically restart the child/worker process if it dies.

backend_idle_timeout

Timeout before we close unused backend connections.

backend_local_error_holddown

When connecting to backends, certain error codes (EADDRNOTAVAIL, EACCESS, EPERM) signal a local resource shortage or configuration issue for which retrying connection attempts may worsen the situation due to the complexity of the operations involved in the kernel. This parameter prevents repeated connection attempts for the configured duration.

backend_remote_error_holddown

When connecting to backends, certain error codes (ECONNREFUSED, ENETUNREACH) signal fundamental connection issues such as the backend not accepting connections or routing problems for which repeated connection attempts are considered useless This parameter prevents repeated connection attempts for the configured duration.

ban_cutoff

Expurge long tail content from the cache to keep the number of bans below this value. 0 disables.

When this parameter is set to a non-zero value, the ban lurker continues to work the ban list as usual top to bottom, but when it reaches the ban_cutoff-th ban, it treats all objects as if they matched a ban and expurges them from cache. As actively used objects get tested against the ban list at request time and thus are likely to be associated with bans near the top of the ban list, with ban_cutoff, least recently accessed objects (the "long tail") are removed.

This parameter is a safety net to avoid bad response times due to bans being tested at lookup time. Setting a cutoff trades response time for cache efficiency. The recommended value is proportional to rate(bans_lurker_tests_tested) / n_objects while the ban lurker is working, which is the number of bans the system can sustain. The additional latency due to request ban testing is in the order of ban_cutoff / rate(bans_lurker_tests_tested). For example, for rate(bans_lurker_tests_tested) = 2M/s and a tolerable latency of 100ms, a good value for ban_cutoff may be 200K.

ban_dups

Eliminate older identical bans when a new ban is added. This saves CPU cycles by not comparing objects to identical bans. This is a waste of time if you have many bans which are never identical.

ban_lurker_age

The ban lurker will ignore bans until they are this old. When a ban is added, the active traffic will be tested against it as part of object lookup. Because many applications issue bans in bursts, this parameter holds the ban-lurker off until the rush is over. This should be set to the approximate time which a ban-burst takes.

ban_lurker_batch

The ban lurker sleeps ${ban_lurker_sleep} after examining this many objects. Use this to pace the ban-lurker if it eats too many resources.

ban_lurker_holdoff

How long the ban lurker sleeps when giving way to lookup due to lock contention.

ban_lurker_sleep

How long the ban lurker sleeps after examining ${ban_lurker_batch} objects. Use this to pace the ban-lurker if it eats too many resources. A value of zero will disable the ban lurker entirely.

between_bytes_timeout

We only wait for this many seconds between bytes received from the backend before giving up the fetch. VCL values, per backend or per backend request take precedence. This parameter does not apply to pipe'ed requests.

cc_command

The command used for compiling the C source code to a dlopen(3) loadable object. The following expansions can be used:

Unknown percent expansion sequences are ignored, and to avoid future incompatibilities percent characters should be escaped with a double percent sequence.

The %d and %D expansions allow passing the parameter's default value to a wrapper script to perform additional processing.

cc_warnings

Warnings used when compiling the C source code with the cc_command parameter. By default, VCL is compiled with the same set of warnings as Varnish itself.

cli_limit

Maximum size of CLI response. If the response exceeds this limit, the response code will be 201 instead of 200 and the last line will indicate the truncation.

cli_timeout

Timeout for the child's replies to CLI requests from the mgt_param.

clock_skew

How much clockskew we are willing to accept between the backend and our own clock.

clock_step

How much observed clock step we are willing to accept before we panic.

connect_timeout

Default connection timeout for backend connections. We only try to connect to the backend for this many seconds before giving up. VCL can override this default value for each backend and backend request.

critbit_cooloff

How long the critbit hasher keeps deleted objheads on the cooloff list.

debug

Enable/Disable various kinds of debugging.

Use +/- prefix to set/reset individual bits:

default_grace

Default grace period. We will deliver an object this long after it has expired, provided another thread is attempting to get a new copy.

default_keep

Default keep period. We will keep a useless object around this long, making it available for conditional backend fetches. That means that the object will be removed from the cache at the end of ttl+grace+keep.

default_ttl

The TTL assigned to objects if neither the backend nor the VCL code assigns one.

experimental

Enable/Disable experimental features.

Use +/- prefix to set/reset individual bits:

feature

Enable/Disable various minor features.

Use +/- prefix to enable/disable individual feature:

fetch_chunksize

The default chunksize used by fetcher. This should be bigger than the majority of objects with short TTLs. Internal limits in the storage_file module makes increases above 128kb a dubious idea.

fetch_maxchunksize

The maximum chunksize we attempt to allocate from storage. Making this too large may cause delays and storage fragmentation.

first_byte_timeout

Default timeout for receiving first byte from backend. We only wait for this many seconds for the first byte before giving up. VCL can override this default value for each backend and backend request. This parameter does not apply to pipe'ed requests.

gzip_buffer

Size of malloc buffer used for gzip processing. These buffers are used for in-transit data, for instance gunzip'ed data being sent to a client.Making this space to small results in more overhead, writes to sockets etc, making it too big is probably just a waste of memory.

gzip_level

Gzip compression level: 0=debug, 1=fast, 9=best

gzip_memlevel

Gzip memory level 1=slow/least, 9=fast/most compression. Memory impact is 1=1k, 2=2k, ... 9=256k.

h2_header_table_size

HTTP2 header table size. This is the size that will be used for the HPACK dynamic decoding table.

h2_initial_window_size

HTTP2 initial flow control window size.

h2_max_concurrent_streams

HTTP2 Maximum number of concurrent streams. This is the number of requests that can be active at the same time for a single HTTP2 connection.

h2_max_frame_size

HTTP2 maximum per frame payload size we are willing to accept.

h2_max_header_list_size

HTTP2 maximum size of an uncompressed header list.

h2_rx_window_increment

HTTP2 Receive Window Increments. How big credits we send in WINDOW_UPDATE frames Only affects incoming request bodies (ie: POST, PUT etc.)

h2_rx_window_low_water

HTTP2 Receive Window low water mark. We try to keep the window at least this big Only affects incoming request bodies (ie: POST, PUT etc.)

h2_rxbuf_storage

The name of the storage backend that HTTP/2 receive buffers should be allocated from.

http1_iovs

Number of io vectors to allocate for HTTP1 protocol transmission. A HTTP1 header needs 7 + 2 per HTTP header field. Allocated from workspace_thread.

http_gzip_support

Clients that do not support gzip will have their Accept-Encoding header removed. For more information on how gzip is implemented please see the chapter on gzip in the Varnish reference.

When gzip support is disabled the variables beresp.do_gzip and beresp.do_gunzip have no effect in VCL.

http_max_hdr

Maximum number of HTTP header lines we allow in {req|resp|bereq|beresp}.http (obj.http is autosized to the exact number of headers). Cheap, ~20 bytes, in terms of workspace memory. Note that the first line occupies five header lines.

http_range_support

Enable support for HTTP Range headers.

http_req_hdr_len

Maximum length of any HTTP client request header we will allow. The limit is inclusive its continuation lines.

http_req_size

Maximum number of bytes of HTTP client request we will deal with. This is a limit on all bytes up to the double blank line which ends the HTTP request. The memory for the request is allocated from the client workspace (param: workspace_client) and this parameter limits how much of that the request is allowed to take up.

http_resp_hdr_len

Maximum length of any HTTP backend response header we will allow. The limit is inclusive its continuation lines.

http_resp_size

Maximum number of bytes of HTTP backend response we will deal with. This is a limit on all bytes up to the double blank line which ends the HTTP response. The memory for the response is allocated from the backend workspace (param: workspace_backend) and this parameter limits how much of that the response is allowed to take up.

idle_send_timeout

Send timeout for individual pieces of data on client connections. May get extended if 'send_timeout' applies.

When this timeout is hit, the session is closed.

See the man page for setsockopt(2) or socket(7) under SO_SNDTIMEO for more information.

listen_depth

Listen queue depth.

lru_interval

Grace period before object moves on LRU list. Objects are only moved to the front of the LRU list if they have not been moved there already inside this timeout period. This reduces the amount of lock operations necessary for LRU list access.

max_esi_depth

Maximum depth of esi:include processing.

max_restarts

Upper limit on how many times a request can restart.

max_retries

Upper limit on how many times a backend fetch can retry.

max_vcl

Threshold of loaded VCL programs. (VCL labels are not counted.) Parameter max_vcl_handling determines behaviour.

max_vcl_handling

Behaviour when attempting to exceed max_vcl loaded VCL.

nuke_limit

Maximum number of objects we attempt to nuke in order to make space for a object body.

pcre2_depth_limit

The recursion depth-limit for the internal match logic in a pcre2_match().

(See: pcre2_set_depth_limit() in pcre2 docs.)

This puts an upper limit on the amount of stack used by PCRE2 for certain classes of regular expressions.

We have set the default value low in order to prevent crashes, at the cost of possible regexp matching failures.

Matching failures will show up in the log as VCL_Error messages.

pcre2_jit_compilation

Use the pcre2 JIT compiler if available.

pcre2_match_limit

The limit for the number of calls to the internal match logic in pcre2_match().

(See: pcre2_set_match_limit() in pcre2 docs.)

This parameter limits how much CPU time regular expression matching can soak up.

ping_interval

Interval between pings from parent to child. Zero will disable pinging entirely, which makes it possible to attach a debugger to the child.

pipe_sess_max

Maximum number of sessions dedicated to pipe transactions.

pipe_timeout

Idle timeout for PIPE sessions. If nothing have been received in either direction for this many seconds, the session is closed.

pool_req

Parameters for per worker pool request memory pool.

The three numbers are:

pool_sess

Parameters for per worker pool session memory pool.

The three numbers are:

pool_vbo

Parameters for backend object fetch memory pool.

The three numbers are:

prefer_ipv6

Prefer IPv6 address when connecting to backends which have both IPv4 and IPv6 addresses.

rush_exponent

How many parked request we start for each completed request on the object. NB: Even with the implict delay of delivery, this parameter controls an exponential increase in number of worker threads.

send_timeout

Total timeout for ordinary HTTP1 responses. Does not apply to some internally generated errors and pipe mode.

When 'idle_send_timeout' is hit while sending an HTTP1 response, the timeout is extended unless the total time already taken for sending the response in its entirety exceeds this many seconds.

When this timeout is hit, the session is closed

shortlived

Objects created with (ttl+grace+keep) shorter than this are always put in transient storage.

sigsegv_handler

Install a signal handler which tries to dump debug information on segmentation faults, bus errors and abort signals.

syslog_cli_traffic

Log all CLI traffic to syslog(LOG_INFO).

tcp_fastopen

Enable TCP Fast Open extension.

tcp_keepalive_intvl

The number of seconds between TCP keep-alive probes. Ignored for Unix domain sockets.

tcp_keepalive_probes

The maximum number of TCP keep-alive probes to send before giving up and killing the connection if no response is obtained from the other end. Ignored for Unix domain sockets.

tcp_keepalive_time

The number of seconds a connection needs to be idle before TCP begins sending out keep-alive probes. Ignored for Unix domain sockets.

thread_pool_add_delay

Wait at least this long after creating a thread.

Some (buggy) systems may need a short (sub-second) delay between creating threads. Set this to a few milliseconds if you see the 'threads_failed' counter grow too much.

Setting this too high results in insufficient worker threads.

thread_pool_destroy_delay

Wait this long after destroying a thread.

This controls the decay of thread pools when idle(-ish).

thread_pool_fail_delay

Wait at least this long after a failed thread creation before trying to create another thread.

Failure to create a worker thread is often a sign that the end is near, because the process is running out of some resource. This delay tries to not rush the end on needlessly.

If thread creation failures are a problem, check that thread_pool_max is not too high.

It may also help to increase thread_pool_timeout and thread_pool_min, to reduce the rate at which treads are destroyed and later recreated.

thread_pool_max

The maximum number of worker threads in each pool.

Do not set this higher than you have to, since excess worker threads soak up RAM and CPU and generally just get in the way of getting work done.

thread_pool_min

The minimum number of worker threads in each pool.

Increasing this may help ramp up faster from low load situations or when threads have expired.

Technical minimum is 5 threads, but this parameter is strongly recommended to be at least 10

thread_pool_reserve

The number of worker threads reserved for vital tasks in each pool.

Tasks may require other tasks to complete (for example, client requests may require backend requests, http2 sessions require streams, which require requests). This reserve is to ensure that lower priority tasks do not prevent higher priority tasks from running even under high load.

The effective value is at least 5 (the number of internal priority classes), irrespective of this parameter.

thread_pool_stack

Worker thread stack size. This will likely be rounded up to a multiple of 4k (or whatever the page_size might be) by the kernel.

The required stack size is primarily driven by the depth of the call-tree. The most common relevant determining factors in varnish core code are GZIP (un)compression, ESI processing and regular expression matches. VMODs may also require significant amounts of additional stack. The nesting depth of VCL subs is another factor, although typically not predominant.

The stack size is per thread, so the maximum total memory required for worker thread stacks is in the order of size = thread_pools x thread_pool_max x thread_pool_stack.

Thus, in particular for setups with many threads, keeping the stack size at a minimum helps reduce the amount of memory required by Varnish.

On the other hand, thread_pool_stack must be large enough under all circumstances, otherwise varnish will crash due to a stack overflow. Usually, a stack overflow manifests itself as a segmentation fault (aka segfault / SIGSEGV) with the faulting address being near the stack pointer (sp).

Unless stack usage can be reduced, thread_pool_stack must be increased when a stack overflow occurs. Setting it in 150%-200% increments is recommended until stack overflows cease to occur.

thread_pool_timeout

Thread idle threshold.

Threads in excess of thread_pool_min, which have been idle for at least this long, will be destroyed.

thread_pool_watchdog

Thread queue stuck watchdog.

If no queued work have been released for this long, the worker process panics itself.

thread_pools

Number of worker thread pools.

Increasing the number of worker pools decreases lock contention. Each worker pool also has a thread accepting new connections, so for very high rates of incoming new connections on systems with many cores, increasing the worker pools may be required.

Too many pools waste CPU and RAM resources, and more than one pool for each CPU is most likely detrimental to performance.

Can be increased on the fly, but decreases require a restart to take effect, unless the drop_pools experimental debug flag is set.

thread_queue_limit

Permitted request queue length per thread-pool.

This sets the number of requests we will queue, waiting for an available thread. Above this limit sessions will be dropped instead of queued.

thread_stats_rate

Worker threads accumulate statistics, and dump these into the global stats counters if the lock is free when they finish a job (request/fetch etc.) This parameters defines the maximum number of jobs a worker thread may handle, before it is forced to dump its accumulated stats into the global counters.

timeout_idle

Idle timeout for client connections.

A connection is considered idle until we have received the full request headers.

This parameter is particularly relevant for HTTP1 keepalive connections which are closed unless the next request is received before this timeout is reached.

timeout_linger

How long the worker thread lingers on an idle session before handing it over to the waiter. When sessions are reused, as much as half of all reuses happen within the first 100 msec of the previous request completing. Setting this too high results in worker threads not doing anything for their keep, setting it too low just means that more sessions take a detour around the waiter.

vary_notice

How many variants need to be evaluated to log a Notice that there might be too many variants.

vcc_allow_inline_c

Allow inline C code in VCL.

vcc_err_unref

Unreferenced VCL objects result in error.

vcc_unsafe_path

Allow '/' in vmod & include paths. Allow 'import ... from ...'.

vcl_cooldown

How long a VCL is kept warm after being replaced as the active VCL (granularity approximately 30 seconds).

vcl_path

Directory (or colon separated list of directories) from which relative VCL filenames (vcl.load and include) are to be found. By default Varnish searches VCL files in both the system configuration and shared data directories to allow packages to drop their VCL files in a standard location where relative includes would work.

vmod_path

Directory (or colon separated list of directories) where VMODs are to be found.

vsl_buffer

Bytes of (req-/backend-)workspace dedicated to buffering VSL records. When this parameter is adjusted, most likely workspace_client and workspace_backend will have to be adjusted by the same amount.

Setting this too high costs memory, setting it too low will cause more VSL flushes and likely increase lock-contention on the VSL mutex.

vsl_mask

Mask individual VSL messages from being logged.

Use +/- prefix in front of VSL tag name to unmask/mask individual VSL messages.

vsl_reclen

Maximum number of bytes in SHM log record.

vsl_space

The amount of space to allocate for the VSL fifo buffer in the VSM memory segment. If you make this too small, varnish{ncsa|log} etc will not be able to keep up. Making it too large just costs memory resources.

vsm_free_cooldown

How long VSM memory is kept warm after a deallocation (granularity approximately 2 seconds).

workspace_backend

Bytes of HTTP protocol workspace for backend HTTP req/resp. If larger than 4k, use a multiple of 4k for VM efficiency.

workspace_client

Bytes of HTTP protocol workspace for clients HTTP req/resp. Use a multiple of 4k for VM efficiency. For HTTP/2 compliance this must be at least 20k, in order to receive fullsize (=16k) frames from the client. That usually happens only in POST/PUT bodies. For other traffic-patterns smaller values work just fine.

workspace_session

Allocation size for session structure and workspace. The workspace is primarily used for TCP connection addresses. If larger than 4k, use a multiple of 4k for VM efficiency.

workspace_thread

Bytes of auxiliary workspace per thread. This workspace is used for certain temporary data structures during the operation of a worker thread. One use is for the IO-vectors used during delivery. Setting this parameter too low may increase the number of writev() syscalls, setting it too high just wastes space. ~0.1k + UIO_MAXIOV * sizeof(struct iovec) (typically = ~16k for 64bit) is considered the maximum sensible value under any known circumstances (excluding exotic vmod use).