Scott Long wrote:
Sven Petai wrote:
On Tuesday 09 May 2006 20:35, Julian Elischer wrote:
Sven Petai wrote:
are there any patches that take the gettimeofday() calls and replace
them with something that is cheap
such as only doing every 10th one and just returning the last value
++ 1
uSec for the other ones..
a ktrace of Mysql shows a LOT of gettimeofday() calls.
well I have actually done that although in a very hackish way that
is suitable *only* for benchmarking, but my goal really was
just to find out if gettimeofday really is the bottleneck.
Basically I just preloaded my version of "buffering" time() over libc's.
That function asked kernel for time only after every 5000 invocations.
I could get away with that for benchmarking since
about 99,99% of gettimeofday() calls from mysqld really come through
time() which has anyway only 1 second resolution and mysqld seems to use
the value for purposes that are not really all that critical, like
stats and safeguard timers.
It's also called far more than 5000 times in a second while benchmark is
running so it should return correct values too.
Anyway, the results really astonished me, what I got for reducing
gettimeofday() calls by ~96% was performance *decrease* of about 4-5%.
I thought something was wrong with my measurement techniques, so I
wrote the buffering time() function that used TSC directly into mysqld,
replaced all the time() calls with that, so that number of
gettimeofday()
syscalls ktrace reported was down to only the initial one, but still
got the same 5% decrease.
I just can't explain it, but can't find any mistakes in methods too.
The library I used is available @
http://bsd.ee/~hadara/debug/mysql4/time_lib_hack.c
Were you testing on SMP, and if so, was Hyperthreading enabled?
Scott
I think that depends on the type of testing, not hardware. I simply
replaced
gettimeofday() syscall with a super simple user space code, for testing
with no time
accuracy, in libc (libc.so.6.test, which is mapped for mysqld and mysql
via libmap --
see details at the end),
and found that code has no effect on "select-only" super-smack test, and has
some impact (7-8%) for super-smack "update-select" test.
The code itself is about 80~90 times faster than gettimeofday syscall.
So, the slow is not mainly caused by timestamp, but I/O and other syscalls.
129 /tmp: ldd /usr/local/libexec/mysqld
/usr/local/libexec/mysqld:
... skipped
libpthread.so.2 => /usr/lib/libthr.so.2 (0x884fc000)
libc.so.6 => /lib/libc.so.6 (0x8850e000)
130 /tmp: super-smack -d mysql
/usr/local/share/super-smack/update-select.smack 10 7000
Query Barrel Report for client smacker
connect: max=4ms min=4ms avg= 4ms from 10 clients
Query_type num_queries max_time min_time q_per_s
select_index 70000 1 0 3125.63
update_index 70000 1 0 3125.63
2.742u 4.058s 0:22.52 30.1% 119+519k 0+0io 0pf+0w
----- edit /etc/libmap.conf and restart mysqld ----------
131 /tmp: ldd /usr/local/libexec/mysqld
/usr/local/libexec/mysqld:
... skiipped
libpthread.so.2 => /usr/lib/libthr.so.2 (0x884fc000)
libc.so.6 => /lib/libc.so.6.test (0x8850e000)
132 /tmp: super-smack -d mysql
/usr/local/share/super-smack/update-select.smack 10 7000
Query Barrel Report for client smacker
connect: max=5ms min=2ms avg= 3ms from 10 clients
Query_type num_queries max_time min_time q_per_s
select_index 70000 1 0 3356.99
update_index 70000 1 0 3356.99
2.634u 4.198s 0:20.97 32.5% 116+506k 0+0io 0pf+0w
133 /tmp: super-smack -d mysql
/usr/local/share/super-smack/update-select.smack 10 7000
Query Barrel Report for client smacker
connect: max=248ms min=1ms avg= 132ms from 10 clients
Query_type num_queries max_time min_time q_per_s
select_index 70000 1 0 3334.93
update_index 70000 1 0 3334.93
2.604u 4.247s 0:21.11 32.4% 115+503k 0+0io 0pf+0w
------------- original gettimeofday code ----------
< #include "SYS.h"
< RSYSCALL(gettimeofday)
--------- replacement code ---
#include <sys/time.h>
static struct timeval tmv_base={$COMPILING_TIME};
#ifndef FAKE_INC
#define FAKE_INC 700 /* 100-times longer as per gettimeofday
cost */
#endif
#define ONE_MEGA 1000000
int
gettimeofday(struct timeval *tp, struct timezone *nouse)
{
tmv_base.tv_usec += FAKE_INC;
if (tmv_base.tv_usec >= ONE_MEGA)
tmv_base.tv_usec -= ONE_MEGA,
++tmv_base.tv_sec;
if (tp) {
*tp = tmv_base;
return 0;
}
return -1;
}
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