High LRU nuked with plenty of cache space available

[MAGNIEN, Thierry]

You're right, file storage is a mmap, but once varnish decides to write to disk, this can result in a very large performance penalty, depending on your disks.

With 12G, I would basically suggest to switch to RAM storage, unless you don't have enough.

Regards,
Thierry

-----Message d'origine-----
De : varnish-misc-bounces+thierry.magnien=sfr.com at varnish-cache.org [mailto:varnish-misc-bounces+thierry.magnien=sfr.com at varnish-cache.org] De la part de Dan Crosta
Envoyé : lundi 28 juillet 2014 17:18
À : varnish-misc at varnish-cache.org
Objet : Re: High LRU nuked with plenty of cache space available

On Mon, Jul 28, 2014 at 9:35 AM, MAGNIEN, Thierry
<thierry.magnien at sfr.com> wrote:
> Hi Dan,
>
> Things you should check first are :
> - varnishlog for "could not get storage" errors

OK -- I'm grepping varnishlog, will let you know if I see anything there.


> - disk activity/performance: maybe varnish cannot get storage because your disks are responding too slowly

I thought the "file" storage class was mmap'd storage, and so writes
should basically hit memory and be periodically background flushed to
disk, is that not true? I'll keep an eye on await in iostat and let
you know how it looks. Is there a particular threshold I should be
watching out for? Is the threshold tunable?


Thanks,
- Dan
MAGNE+IC
Dan Crosta | Director, Engineering


On Mon, Jul 28, 2014 at 9:35 AM, MAGNIEN, Thierry
<thierry.magnien at sfr.com> wrote:
> Hi Dan,
>
> Things you should check first are :
> - varnishlog for "could not get storage" errors
> - disk activity/performance: maybe varnish cannot get storage because your disks are responding too slowly
>
> Regards,
> Thierry
>
> -----Message d'origine-----
> De : varnish-misc-bounces+thierry.magnien=sfr.com at varnish-cache.org [mailto:varnish-misc-bounces+thierry.magnien=sfr.com at varnish-cache.org] De la part de Dan Crosta
> Envoyé : samedi 26 juillet 2014 14:58
> À : varnish-misc at varnish-cache.org
> Objet : High LRU nuked with plenty of cache space available
>
> We're running varnish (3.0.4) like:
>
> /usr/sbin/varnishd -P /var/run/varnish.pid -a :80 -f
> /etc/varnish/default.vcl -T 127.0.0.1:8080 -t 2628000 -w 50,1000,120
> -u varnish -g varnish -S /etc/varnish/secret -s
> file,/srv/varnish/varnish_storage.bin,12G -p thread_pools=4 -p
> http_gzip_support=off -p saintmode_threshold=0 -p
> http_range_support=off
>
> varnishstat shows the following for SMF:
>
> SMF.s0.c_req            45213612       317.34 Allocator requests
> SMF.s0.c_fail           14781854       103.75 Allocator failures
> SMF.s0.c_bytes      2056164147200  14431753.97 Bytes allocated
> SMF.s0.c_freed      2052654858240  14407123.06 Bytes freed
> SMF.s0.g_alloc            856760          .   Allocations outstanding
> SMF.s0.g_bytes        3509288960          .   Bytes outstanding
> SMF.s0.g_space        9375612928          .   Bytes available
> SMF.s0.g_smf              950080          .   N struct smf
> SMF.s0.g_smf_frag          93319          .   N small free smf
> SMF.s0.g_smf_large             1          .   N large free smf
>
> which I interpret to mean that ~9G of the 12G are available ("bytes
> available") for use by the cache.
>
> However, when our applications service the API through varnish at a
> rate of about 150 QPS, we see that n_lru_nuked increases at about the
> same rate, the result of which is that the n_object counter stays more
> or less constant. The cache has only been running for a few days, so I
> don't believe TTL is to blame here, since we use -t with about 30 days
> (assuming the value there is in seconds, which the docs seem to
> imply).
>
> I have tried setting a granularity on the storage, which did not seem
> to have any impact. I'm looking for other suggestions or things to
> try, as intuition and the stats seem to suggest we should be able to
> store a lot more objects before things start to get nuked.
>
> Thanks,
> - Dan
>
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> varnish-misc at varnish-cache.org
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