Shanti Subramanyam wrote:
Thanks Will. At this point, the PHP app is only caching the home page.
We are wondering whether to even do the Event Detail page as the load
on the database has been drastically cut down just from the home page
caching.
It's a dilemma - if we cache too much, there is no load on the db. If
we cache too little, there is nothing much in memcached. Of course, if
we run a much larger scale (say, 10's of systems for the web tier),
then I'm sure we'll see increasing load on both tiers. But practically
speaking, we need to be able to run a reasonable configuration.
IMO reducing the load on the DB should be a high order goal of any
testing. The DB is a bottleneck, anything that can be cached should be
cached and if you want to test DB performance ramp up the number of
users until the DB struggles and then partition it and then partition it
some more.
Akara has another idea to use memcached more heavily, while at the
same time not reducing the db load. Namely, cache the thumbnails in
it. This will also reduce the load on the filestore (which currently
is quite heavily stressed for the PHP app). But this strategy won't
work for the rails app will it ? I believe you're serving all static
files out of the proxy server ?
I'm looking to do as Will suggests, serve the static files from Nginx
and cache them with the Nginx memcached module. At the moment I'm in
Nginx/Thin heaven as they work so well together and I've barely
scratched the surface of what's possible.
Amanda
Would love to hear what others think as well.
Shanti
William Sobel wrote:
On Jan 22, 2009, at 5:11 PM, Shanti Subramanyam wrote:
Can you please elaborate on what exactly is cached ? How is the
cache managed (in terms of timeouts etc.) ?
From the original writeup:
Cache Strategy for Web20Kit
Home Page
The home page will be cached in two forms:
1. Cached as a whole page accessed by users arriving at the site and
users that are not logged on.
2. Cached as a page fragment, just for the content part. The page
will be constructed from the dynamic header which contains the user
name of the current user and the cached content fragment.
3. Paginations – these will be cached up to 5 pages. It is less
likely for users to search for events beyond the fifth page.
Expiration and re-generation
The home page will expire every 120 seconds. Then the page will be
re-generated by one of the first requests arriving after the
expiration. To prevent all requests arriving after the expiration
from re-generating, thus causing a stampede phenomenon, we will use a
lock/semaphore control mechanism as follows:
1. The home page and/or home page fragment is cached with no timeout
or a very large timeout (in the order of magnitude of days) in
memcached.
2. For each cached page, a small semaphore object is placed into
memcached with a timeout of 120 seconds – the regeneration cycle.
3. After accessing the page/fragment in the cache and sending the
response to the user, the cache client (web server) checks to see
whether the semaphore is there or has timed out. If it is not there
(timed out), the client will attempt to re-generate the page or
fragment.
4. To prevent a stampede, the client ‘adds’ a lock entry into the
cache. If the add succeeds, this thread has the lock. The lock times
out after 20 seconds using the memcached timeout mechanism. This
prevents a thread to hold a lock indefinitely.
5. After obtaining the lock, the thread generates the page or
fragment and replaces the copy in memcached.
6. Then the generating thread places a new semaphore object with the
same timeout period and removes the lock object.
Event Detail Page
The event detail page is cached as both content and, if not logged
on, the whole page as well.
Expiration and re-generation
Event detail page cache entries have a time out of 30 seconds using
the cache timeout mechanism of memcached. Thus only frequently
accessed events will remain in the cache. The load generator will
need to be designed to access event detail pages in a non-uniform
manner, too. We will use a locking mechanism for the event detail
page in a similar manner to the home page. However, we will not use
an expiry semaphore and let the page expire from the cache as a
whole. Access to the entry should however renew the expiry time so
that frequently accessed events will stay in cache. The mechanism
will work as follows:
1. The event detail page and fragment is cached with a timeout of 30
seconds.
2. As a cache client needs to access the entry, it will try to read
the entry from the cache. If the entry is available, it will extend
the cache timeout. Otherwise, the event detail page is generated from
the database.
3. To regenerate the page and prevent stampede, the client ‘adds’ a
lock entry into the cache. If the add succeeds, this thread has the
lock. The lock times out after 20 seconds using the memcached timeout
mechanism. This prevents a thread to hold a lock inidefinitely.
4. After obtaining the lock, the thread proceeds with generating the
page. After completion, the page gets placed into the cache and the
lock gets removed from memcached.
5. If we do not get the lock (add fails). We stay in a loop, sleep
for 200ms, and check/re-check whether the page matches. We keep
checking till a timeout of 5 seconds (25 iterations).
6. The attendee list and comments/rating fragments of this page is
cached in the same manner. Those sections will be re-generated while
holding a lock object in the same manner. They will be regenerated if
the fragment is not in the cache, and on or after updating of those
fragments (i.e. somebody makes a comment or signed up to attend this
event).
Other Pages
At this point, none of the other pages and/or their fragments are
cached. Most of the other pages are accessed at low frequency with
the exception of the tag search page. The tag search page is the next
candidate for caching and pre-generation. The caching strategy is
still to be determined.
Page Caches with Ruby on Rails
Ruby on Rails does not natively use memcached for whole page caches.
It can do so with caching page fragments. Instead, it will generate
static pages as files and the request will be routed to the
corresponding file that represents a fully rendered page.
The Ruby on Rails implementation of Web20Kit will use the native
Rails mechanism for full page caches. Expirations result in a call to
remove the file and follow the same expiry policy defined for each
page, above. The file must be removed as the page cache expires,
either by a request arriving after expiry, or by a background job.
Cheers,
- Will Sobel
