Performance when changing the length of a keys value. (It gets really bad.)

[Alex Stapleton]

I am running under Debian 3.0 with Linux 2.4.23, Quad 2.4 GHz P4  
Xeon. 2GB RAM. My memcached is running with these options

/usr/bin/memcached -m 128 -p 11211 -u root -c 1024

Performance is generally rather good. I can push 6000-7000 requests/s  
doing over 100MB/s of data in throughput. However! In my test code,  
if I change the data block size from 8192 bytes, to 1024 bytes,  
without restarting the server, Performance suddenly becomes hideously  
slow. So slow in fact, the tests which where taking only 2-3 seconds  
to complete, now take over 10 times longer, reducing my requests a  
second from several thousand, to below 10!

Does anyone have any suggestions as to why this might be? I am  
writing approximately 80 MB of data to the server in my tests, using  
the same keys between tests, so they should just get overwritten,  
right? I expect it is due to memcached suddenly deciding to re-malloc  
or free memory when I reduce the data size.

Here is my test script. It is written in Python using the danga  
memcached module. I hope nothing mangles the formatting.

import time
import memcache

mc = memcache.Client(['127.0.0.1:11211'], debug=0)

iters = 100
data_len = 4098

data = "B" * data_len

s = time.time()

for i in xrange(1,iters):
         mc.set(i, data)

e = time.time()
took = (e-s)

print "Took %s seconds" % (took)
print "Wrote at least %s MB of data" % ((data_len*iters)/1024/1024)
print "%s t/s" % (iters/took)
print "%s s/t" % (took/iters)
print "%s MB/s" % ((data_len*iters/took)*2/1024/1024)