X-Git-Url: http://git.lttng.org./?a=blobdiff_plain;f=genevent-new%2FREADME;h=7b16fcdf406a01815ce33b742e58ed7c31d07004;hb=d3cd9e86e2c959b5cf35ff4c47a70822f78cb36e;hp=5c9fb4e90f890e866aa9fd8f60f4ba0662b33d7f;hpb=3583026d9a76dc3c9402f95a41b2c54dee63b8c8;p=lttv.git
diff --git a/genevent-new/README b/genevent-new/README
index 5c9fb4e9..7b16fcdf 100644
--- a/genevent-new/README
+++ b/genevent-new/README
@@ -12,6 +12,57 @@ There are several files in the directory:
In fact, crc32.tab, parser.c and parser.h are the same files as
those in LTT library.
+Note about strings :
+There are three methods to write strings in genevent, each suitable and
+efficient for a particular case. They are explained here from the fastest
+to the slowest.
+1 - The C code presents a fixed size string.
+ For example, you find :
+ char mystring[10];
+ as string definition.
+
+ you must then define it as an array of char :
+
+
+ Note, however, that you might not want to declare a fixed size for trace size
+ and unnecessary copy matters.
+
+ For instance, on a 32 bits architecture, copying a n bytes array takes
+ approximately* n/4 memory read and write, for n/2 memory operations.
+
+ Using the slower method described in (3), with a strlen and memcpy, where
+ "u" is the number of used caracters, takes u+1 reads for the strlen, and
+ approximately* (u+1)/4 read and write for the memcpy, for a total of :
+ (3/2)*(u+1) memory access.
+
+ So, if (n/2) > (3/2)*(u+1), or : n > 3*u+3
+ where n is the size of the array
+ u is the average number of used caracters (excluding the \0)
+ it becomes faster to use the method number 3 with strlen.
+
+2 - The C code presents a variable size string together with its
+ size.
+
+ A typical use for this case is filenames in the Linux kernel. The
+ dentry strucure has a d_name member, which is a struct qstr containing
+ a unsigned int len and const unsigned char *name.
+
+ you must use a sequence to declare this efficiently :
+
+
+3 - The C code presents a \0 terminated string.
+
+ This is the slowest, but most convenient way to declare a string. You are
+ discouraged to use it when options 1 or 2 are available. It will dynamically
+ calculate the string length (byte by byte read) and only afterward do a
+ memcpy.
+
+ Note that, as explained in 1, if n > 3*u+3, it becomes faster to use this
+ method instead of copying the whole fixed size array.
+
+ Declare like this :
+
+
Here is a brief description of how to use genevent.
make
@@ -37,3 +88,8 @@ cp ltt-facility-loader-yourfacility.c ltt-facility-loader-yourfacility.h \
/usr/src/linux-2.6.12-rc4-mm2-lttng-0.2/include/linux/ltt/ltt-facility-yourfacility.h
+
+* The approximation comes from the fact that copies of number of caracters non
+ multiple of the architecture size takes more operations (maximum of :
+ (architecture size (in bytes) - 1) operations).
+