Sorry, I missed this message a couple of days ago. Answers inline...
From: Vishal1 Sharma
Sent: 11 March 2020 12:37
To: 'vpp-dev@lists.fd.io' <vpp-dev@lists.fd.io<mailto:vpp-dev@lists.fd.io>>
Subject: Storing a struct as a value in BiHash
Hello all,
I’m new to using vpp-infra and I’m currently trying to use hash table in order
to store some structs as value. Thanks to Dave Barach, I’ve got a sample
program for inserting a value and fetching that. However, I have some queries
regarding the program. Given below is the program:
#include <stdio.h>
#include "vppinfra/bihash_8_8.h"
#include "vppinfra/bihash_template.c"
//This is my structure that I want to save as value.
typedef struct
{
int a;
char b[100];
char c;
} design_t;
typedef struct
{
/* Pool of designs */
design_t *designs;
/* design by key */
uword *design_by_key;
} test_main_t;
test_main_t test_main;
int main (int argc, char **argv)
{
test_main_t *tm = &test_main;
design_t *elt;
uword *p;
/* Initialize the memory allocator */
clib_mem_init (0, 128<<20);
/* Create a 32-bit key hash table with a 32-bit value */
tm->design_by_key = hash_create (0, sizeof (32));
/*
* Create an element
* Remember that pools are vectors, which can/will MOVE
* when resized. Always memorize pool indices, NOT pointers.
*/
pool_get (tm->designs, elt);
memset (elt, 0, sizeof (*elt)); /* pool_get doesn't zero memory */
elt->a = 42;
strncpy (elt->b, "Vishal Sharma", sizeof (elt->b));
elt->c = 'v';
/* Add the pool element INDEX to the hash table */
hash_set (tm->design_by_key, 42, elt - tm->designs);
/* For fun, look up the design_t we put into the hash table */
p = hash_get (tm->design_by_key, 42);
ASSERT (p);
elt = pool_elt_at_index (tm->designs, p[0]);
ASSERT (elt->a == 42);
exit (0);
}
I’m compiling this above program by: gcc -o exe prog.c -lvppinfra -I ../ -ggdb3
-I ./vpp/build-root/build-vpp_debug-native/vpp/
Given below are my queries:
1. I didn’t understand the purpose of “test_main_t”. Why is it required?
If you look at vlib/vnet/plugins, you’ll see this coding pattern all over the
place. Typically, one ends up with faster object code as a result. It’s not
“required,” but it is pretty effective.
1. By calling clib_mem_init(), some memory allocation is happening. What
will happen if my usage exceeds that? Will it automatically resize?
Clib_mem_init(0, <size>) allocates <size> worth of virtual space, and sets the
heap so that it will expand as needed. If you pass a chunk of memory (e.g.
acquired by calling mmap) the heap will be fixed-size.
1. By looking at “pool_get (tm->designs, elt);” it seems that we’re getting
some memory for ‘elt’ from ‘tm->designs’ which was described as ‘pool of
desgins’. But when did we allocate the memory for this ‘pool of designs’? Do I
have to call ‘pool_get()’ every time when I have to insert a key-value pair in
the hash table?
Take a look at .../src/vppinfra/pool.h. Pools are dynamic arrays (vectors) with
headers. The header has a free-element bitmap, and a freelist. All of the pool
structures are dynamic, no need to manually allocate the pool. Fixed-size,
preallocated pools are also supported; no need to go there in this case.
The only “gotcha” with pools: they ARE dynamic, so memorizing the address of a
pool element is a bad mistake.
Call pool_get to acquire a new element from a pool. In most cases, the vector
will not expand, so the operation is extremely fast.
1. When we are setting the key-value in the hash table by calling
‘hash_set()’, I didn’t understand the 3rd argument being passed. Why is it ‘elt
- tm->designs’ and not just ‘elt’? What’s being done by the subtraction here?
Always memorize pool indices. Never memorize a pointer to the ith element of a
pool. Again, pools are dynamic. When a pool expands, we copy the current
contents to a new, larger chunk of allocated memory. We free the old memory.
Sticking pointers to pool elements into a hash table is a bad mistake, and
leads to catastrophic failure when the old memory is reallocated.
I’ll be grateful if somebody can answer these queries.
Thanks and regards,
Vishal Sharma
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