I need to achieve the following:
Process preemption should be done in every time quantum (measured in clock ticks) instead of every clock tick
In order to achieve this, I have modified the yield() function in proc.c
void
yield(void)
{
struct proc *p = myproc();
acquire(&ptable.lock); //DOC: yieldlock
p->state = RUNNABLE;
p->timeslice--;
if(!p->timeslice)
sched();
release(&ptable.lock);
}
I have added the field int timeslice in struct proc. I timeslice is initialized in scheduler(), every time a process is scheduled
void
scheduler(void)
{
struct proc *p;
struct cpu *c = mycpu();
c->proc = 0;
for(;;){
// Enable interrupts on this processor.
sti();
// Loop over process table looking for process to run.
acquire(&ptable.lock);
for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){
if(p->state != RUNNABLE)
continue;
// Switch to chosen process. It is the process's job
// to release ptable.lock and then reacquire it
// before jumping back to us.
c->proc = p;
switchuvm(p);
p->state = RUNNING;
p->timeslice = QUANTA;
swtch(&(c->scheduler), p->context);
switchkvm();
// Process is done running for now.
// It should have changed its p->state before coming back.
c->proc = 0;
}
release(&ptable.lock);
}
}
But I am getting the following error in xv6 shell
xv6...
cpu1: starting 1
cpu0: starting 0
sb: size 1000 nblocks 941 ninodes 200 nlog 30 logstart 2 inodestart 32 bmap start 58
unexpected trap 14 from cpu 1 eip 801059d2 (cr2=0x3920)
lapicid 1: panic: trap
80105cdf 801059c9 0 0 0 0 0 0 0 0
Please help me fix the error. Thanks in advance ...
You should not change the process state if you don't call sched().
void
yield(void)
{
acquire(&ptable.lock);
struct proc *p = myproc();
p->timeslice--;
if(!p->timeslice ) {
acquire(&ptable.lock);
p->state = RUNNABLE;
sched();
}
release(&ptable.lock);
}
Note:
In code above, you might acquire and release the process table in the if body, but I'm not sure that it's save to change p->timeslice if process table is not locked.
While going through the implementation of the DPDK MPSC (multi-produce & single-consumer) Ring Buffer API, i found the code to move the head of the producer for inserting new elements in the Ring buffer. The function is as follows :
static __rte_always_inline unsigned int
__rte_ring_move_prod_head(struct rte_ring *r, unsigned int is_sp,
unsigned int n, enum rte_ring_queue_behavior behavior,
uint32_t *old_head, uint32_t *new_head,
uint32_t *free_entries)
{
const uint32_t capacity = r->capacity;
uint32_t cons_tail;
unsigned int max = n;
int success;
*old_head = __atomic_load_n(&r->prod.head, __ATOMIC_RELAXED);
do {
/* Reset n to the initial burst count */
n = max;
/* Ensure the head is read before tail */
__atomic_thread_fence(__ATOMIC_ACQUIRE);
/* load-acquire synchronize with store-release of ht->tail
* in update_tail.
*/
cons_tail = __atomic_load_n(&r->cons.tail,
__ATOMIC_ACQUIRE);
/* The subtraction is done between two unsigned 32bits value
* (the result is always modulo 32 bits even if we have
* *old_head > cons_tail). So 'free_entries' is always between 0
* and capacity (which is < size).
*/
*free_entries = (capacity + cons_tail - *old_head);
/* check that we have enough room in ring */
if (unlikely(n > *free_entries))
n = (behavior == RTE_RING_QUEUE_FIXED) ?
0 : *free_entries;
if (n == 0)
return 0;
*new_head = *old_head + n;
if (is_sp)
r->prod.head = *new_head, success = 1;
else
/* on failure, *old_head is updated */
success = __atomic_compare_exchange_n(&r->prod.head,
old_head, *new_head,
0, __ATOMIC_RELAXED,
__ATOMIC_RELAXED);
} while (unlikely(success == 0));
return n;
}
The load and compare exchange of the producer's head is done using __ATOMIC_RELAXED memory ordering. Isn't this a problem when multiple producers from different threads produce to the queue. Or am I missing something?
https://doc.dpdk.org/guides/prog_guide/ring_lib.html describes the basic mechanism that DPDK uses for implementing the Ring buffer.
I am using a pressure sensor D6F-PH to measure the pressure difference. This is my Arduino code that I wrote to get the values from the sensor.
#include "Wire.h"
#define addrs 0x6C // I2C bus address
int P;
int I;
float T;c
int initialize(int i2c_addr)
{
//INITIALIZATION AFTER POWER UP
Wire.beginTransmission(i2c_addr);
Wire.write(0x0B);
Wire.write(0x00);
int x = Wire.endTransmission();
return x;
}
int pressure(int i2c_addr)
{
//MCU MODE
Wire.beginTransmission(i2c_addr);
Wire.write(0x00);
Wire.write(0xD0); // reg 0 - address register high byte
// Wire.write(0x51); // reg 1 - address register low byte
Wire.write(0x40); // reg 1 - address register low byte
Wire.write(0x18); // reg 2 - serial control register - indicate # bytes among others (page 7 bottom)
Wire.write(0x06); // reg 3 - value to be written to SENS control register
int x = Wire.endTransmission();
delay(33);
//WRITE
Wire.beginTransmission(i2c_addr);
Wire.write(0x00);
Wire.write(0xD0);
Wire.write(0x51);
Wire.write(0x2C);
x = Wire.endTransmission();
//READ
Wire.beginTransmission(i2c_addr);
Wire.write(0x07);
x = Wire.endTransmission();
Wire.requestFrom(i2c_addr, 2);
byte hibyte = Wire.read();
byte lobyte = Wire.read();
long raw = word( hibyte, lobyte);
//Serial.print("raw pressure:\t ");
Serial.println(raw);
// D6F-PH5050AD3 ==> rangeMode=500 ==> int rd_pressure = ((raw - 1024) * rangeMode * 2 / 60000L) - rangeMode
// D6F-PH0505AD3 ==> rangeMode=50 ==> int rd_pressure = ((raw - 1024) * rangeMode * 2 / 60000L) - rangeMode
// D6F-PH0025AD1 ==> rangeMode=250 ==> int rd_pressure=(raw - 1024) * rangeMode / 60000L
//int rangeMode = 50;
int rangeMode = 250;
int rd_pressure = (raw - 1024) * rangeMode / 60000L;
//int rd_pressure = ((raw - 1024) * rangeMode * 10/60000L) - rangeMode;
return rd_pressure;
}
float temperature(int i2c_addr)
{
//MCU MODE
Wire.beginTransmission(i2c_addr);
Wire.write(0x00);
Wire.write(0xD0); // reg 0 - address register high byte
// Wire.write(0x51); // reg 1 - address register low byte
Wire.write(0x40); // reg 1 - address register low byte
Wire.write(0x18); // reg 2 - serial control register - indicate # bytes among others (page 7 bottom)
Wire.write(0x06); // reg 3 - value to be written to SENS control register
int x = Wire.endTransmission();
delay(33);
//WRITE
Wire.beginTransmission(i2c_addr);
Wire.write(0x00);
Wire.write(0xD0);
Wire.write(0x61);
Wire.write(0x2C);
x = Wire.endTransmission();
//READ
Wire.beginTransmission(i2c_addr);
Wire.write(0x07);
x = Wire.endTransmission();
Wire.requestFrom(i2c_addr, 2);
byte hibyte = Wire.read();
byte lobyte = Wire.read();
long raw = word( hibyte, lobyte);
//Serial.print("raw temperature:\t ");
//Serial.println(raw);
int temp = round((float)(raw - 10214) / 3.739); // this is the temperature multiplied by 10...
return (temp / 10.0); // ...and the function returs the float
temperature with 0.1°C resolution
}
void setup()
{ // Open serial communications
Wire.begin();
Serial.begin(9600);
I = initialize (addrs); // start wire connection
}
void loop()
{
P = pressure(addrs);
Serial.print("pressure:\t ");
Serial.println(P);
T = temperature(addrs);
Serial.print("temperature:\t ");
Serial.println(T);
delay(300); //delay for 30 seconds
}
I can measure the pressure and the temperature from the arduino uno board using the Serial Monitor.
When I try to Retrieve the data using MATLAB I am unable to do so.
s=serial('COM6','BaudRate',9600);
fopen(s);
This is the error message that I get when I try to open the port : Error using serial/fopen (line 72)
Open failed: Cannot connect to the COM2 port. Possible reasons are another
application is connected to the port or the port does not exist.
I have checked the COM port number and also used the delete(instrfindall); command but to no avail.
Please help
Resolved. I needed to close the Serial Monitor before creating a connection with MATLAB.
I need to implement my own packets to send over UDP. I decided that I would do this by sending a char buffer which has the sequence number, checksum, size, and the data of the packet which is bytes from a file. The string i'm sending separates each field by a semicolon. Then, when I receive the string (which is my packet) I want to extract each felid, use them accordingly (the sequence number, size, and checksum) and write the bytes to a file. So far I have wrote a method to create 100 packets, and I'm trying to extract and write the bytes to a file (I'm not doing it in the receiver yet, first I'm testing the parsing in the sender). For some reason, the bytes written to my file are incorrect and I'm getting "JPEG DATATSTREAM CONTAINS NO IMAGE" error when I try to open it.
struct packetNode{
char packet[1052]; // this is the entire packet data including the header
struct packetNode *next;
};
This is how I'm creating my packets:
//populate initial window of size 100
for(i = 0; i < 100; i++){
memset(&data[0], 0, sizeof(data));
struct packetNode *p; // create packet node
p = (struct packetNode *)malloc(sizeof(struct packetNode));
bytes = fread(data, 1, sizeof(data), fp); // read 1024 bytes from file into data buffer
int b = fwrite(data, 1, bytes, fpNew);
printf("read: %d\n", bytes);
memset(&p->packet[0], 0, sizeof(p->packet));
sprintf(p->packet, "%d;%d;%d;%s", s, 0, numPackets, data); // create packet
//calculate checksum
int check = checksum8(p->packet, sizeof(p->packet));
sprintf(p->packet, "%d;%d;%d;%s", s, check, numPackets, data); //put checksum in packet
s++; //incremenet sequence number
if(i == 0){
head = p;
tail = p;
tail->next = NULL;
}
else{
tail->next = p;
tail = p;
tail->next = NULL;
}
}
fclose(fp);
and this is where I parse and write the bytes to a file:
void test(){
FILE *fpNew = fopen("test.jpg", "w");
struct packetNode *ptr = head;
char *tokens;
int s, c, size;
int i = 0;
char data[1024];
while(ptr != NULL){
memset(&data[0], 0, sizeof(data));
tokens = strtok(ptr->packet,";");
s = atoi(tokens);
tokens = strtok(NULL, ";");
c = atoi(tokens);
tokens = strtok(NULL, ";");
size = atoi(tokens);
tokens = strtok(NULL, ";");
if(tokens != NULL)
strcpy(data, tokens);
printf("sequence: %d, checksum: %d, size: %d\n", s,c,size);
int b = fwrite(data, 1, sizeof(data), fpNew);
ptr = ptr->next;
i++;
}
fclose(fpNew);
}
Since there is transfer of binary data, a JPEG stream, this data cannot be treated as a string. It's better to go all binary. For instance, instead of
sprintf(p->packet, "%d;%d;%d;%s", s, 0, numPackets, data); // create packet
you would do
sprintf(p->packet, "%d;%d;%d;", s, 0, numPackets);
memcpy(&p->packet[strlen(p->packet)], data, bytes);
but this leads to parsing problems: we would need to change this:
tokens = strtok(NULL, ";");
if(tokens != NULL)
strcpy(data, tokens);
to something like this:
tokens += 1 + ( size < 10 ? 1 : size < 100 ? 2 : size < 1000 ? 3 : size < 10000 ? 4 : 5 );
memcpy(data, tokens, sizeof(data));
#Binary Protocol
It's easier to use a binary packet:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#pragma push(pack,1)
typedef struct Packet {
int seq, maxseq, size;
unsigned short cksum;
unsigned char payload[];
} Packet;
#pragma pop(pack)
typedef struct PacketNode{
struct PacketNode * next;
Packet packet;
} PacketNode;
PacketNode * allocPacketNode(int maxPayloadSize) {
void * ptr = malloc(sizeof(PacketNode) + maxPayloadSize); // FIXME: error checking
memset(ptr, 0, sizeof(PacketNode) + maxPayloadSize); // mallocz wouldn't cooperate
return (PacketNode*) ptr;
}
PacketNode * prepare(FILE * fp, int fsize, int chunksize)
{
PacketNode * head = allocPacketNode(chunksize);
PacketNode * pn = head;
int rd, seq = 0;
int maxseq = fsize / chunksize + ( fsize % chunksize ? 1 : 0 );
while ( ( rd = fread(pn->packet.payload, 1, chunksize, fp ) ) > 0 )
{
printf("read %d bytes\n", rd);
pn->packet.seq = seq++;
pn->packet.maxseq = maxseq;
pn->packet.size = rd + sizeof(Packet);
pn->packet.cksum = 0;
pn->packet.cksum = ~checksum(&pn->packet, pn->packet.size);
if ( rd == chunksize )
pn = pn->next = allocPacketNode(chunksize);
}
return head;
}
int checksum(unsigned char * data, int len)
{
int sum = 0, i;
for ( i = 0; i < len; i ++ )
sum += data[i];
if ( sum > 0xffff )
sum = (sum & 0xffff) + (sum>>16);
return sum;
}
void test( PacketNode * ptr ) {
FILE *fpNew = fopen("test.jpg", "w");
while (ptr != NULL)
{
printf("sequence: %d/%d, checksum: %04x, size: %d\n",
ptr->packet.seq,
ptr->packet.maxseq,
ptr->packet.cksum,
ptr->packet.size - sizeof(Packet)
);
int b = fwrite(ptr->packet.payload, ptr->packet.size - sizeof(Packet), 1, fpNew);
ptr = ptr->next;
}
fclose(fpNew);
}
void fatal( const char * msg ) { printf("FATAL: %s\n", msg); exit(1); }
int main(int argc, char** argv)
{
if ( ! argv[1] ) fatal( "missing filename argument" );
FILE * fp = fopen( argv[1], "r" );
if ( ! fp ) fatal( "cannot open file" );
fseek( fp, 0, SEEK_END );
long fsize = ftell(fp);
fseek( fp, 0, SEEK_SET );
printf("Filesize: %d\n", fsize );
test( prepare(fp, fsize, 1024) );
}
The #pragma push(pack,1) changes how the compiler aligns fields of the struct. We want them to be compact, for network transport. Using 1 is byte-aligned. The #pragma pop(pack) restores the previous setting of the pack pragma.
A note on the checksum method
First we calculate the sum of all the bytes in the packet:
int sum = 0, i;
for ( i = 0; i < len; i ++ )
sum += data[i];
Since the packet uses an unsigned short (16 bits, max value 65535 or 0xffff) to store the checksum, we make sure that the result will fit:
if ( sum > 0xffff ) // takes up more than 16 bits.
Getting the low 16 bits of this int is done using sum & 0xffff, masking out everything but the low 16 bits. We could simply return this value, but we would loose the information from higher checksum bits. So, we will add the upper 16 bits to the lower 16 bits. Accessing the higher 16 bits is done by shifting the int to the right 16 bits, like so: sum >> 16. This is the same as sum / 65536, since 65536 = 216 = 1 << 16.
sum = (sum & 0xffff) + (sum>>16); // add low 16 bits and high 16 bits
I should note that network packet checksums are usually computed 2 bytes (or 'octets' as they like to call them there) at a time. For that, the data should be cast to an unsigned short *, and len should be divided by 2. However! len may be odd, so in that case we'll need to take special care of the last byte. For instance, assuming that the maximum packet size is even, and that the len argument is always <= max_packet_size:
unsigned short * in = (unsigned short *) data;
if ( len & 1 ) data[len] = 0; // make sure last byte is 0
len = (len + 1) / 2;
The rest of the checksum method can remain the same, except that it should operate on in instead of data.
I am implementing the recvall() function to be sure that the data is completely sent. Also I modified the send() function to sendall() like this:
int sendall (int consocket, char* buf, int* len)
{
int total = 0;
int bytesleft = *len; // how many we have left to send
int n;
while(total < *len) {
n = send(consocket, buf+total, bytesleft, 0);
if (n == -1) { break; }
total += n;
bytesleft -= n;
*len = total; // return number actually sent here
return n==-1?-1:0; // return -1 on failure, 0 on success
}
}
How can I implement recvall()? Say I sent from the server a struct of 14 bytes and I check in the client and get 12 bytes.. now in an unreliable situation how should I manage to get the other two bytes... I have spent time trying... any help welcomed.
You may loop over a select with an appropriate timeout . Select will wait until new data is received or until time is elapsed or an error occurs. This gives you a greater control than just blocing on a recv.