sys_poll spends around 1/2 of the execution time allocating / freeing a
few bytes temporary memory.
The attached patch tries to avoid these allocation by using the stack -
usually only a few bytes are needed, kmalloc is used for the rest.
The result: one poll of stdin is down from 1736 cpu ticks to 865 cpu
ticks (Pentium II/350 SMP, SMP kernel)
select() should also be faster, but I haven't written a micro-benchmark
for select.
Please test it,
-- Manfred --------------B640E89F46A42E17A441425D Content-Type: text/plain; charset=us-ascii; name="patch-poll" Content-Transfer-Encoding: 7bit Content-Disposition: inline; filename="patch-poll"// $Header$ // Kernel Version: // VERSION = 2 // PATCHLEVEL = 4 // SUBLEVEL = 0 // EXTRAVERSION = --- 2.4/fs/select.c Thu Nov 16 21:54:18 2000 +++ build-2.4/fs/select.c Fri Jan 5 13:46:30 2001 @@ -24,12 +24,6 @@ #define ROUND_UP(x,y) (((x)+(y)-1)/(y)) #define DEFAULT_POLLMASK (POLLIN | POLLOUT | POLLRDNORM | POLLWRNORM) -struct poll_table_entry { - struct file * filp; - wait_queue_t wait; - wait_queue_head_t * wait_address; -}; - struct poll_table_page { struct poll_table_page * next; struct poll_table_entry * entry; @@ -52,11 +46,36 @@ * poll table. */ +/* + * Memory free and alloc took a significant part of the total + * sys_poll()/sys_select() execution time, thus I moved several + * structures on the stack: + * - sys_select has a 192 byte (enough for 256 fds) buffer on the stack. + * Please avoid selecting more than 5000 descriptors + * (kmalloc > 4096 bytes), and you can't select + * more than 170.000 fds (kmalloc > 128 kB) + * - sys_poll stores the first 24 file descriptors on the + * stack. If more than 24 descriptors are polled, then + * additional memory is allocated, but the first 24 descriptors + * always lie on the stack. + * - the poll table contains 8 wait queue entries. This means that no dynamic + * memory allocation is necessary for the wait queues if one of the first + * 8 file descriptors has new data. + * <manfred@colorfullife.com> + */ + void poll_freewait(poll_table* pt) { struct poll_table_page * p = pt->table; + struct poll_table_entry * entry; + entry = pt->internal + pt->nr; + while(pt->nr > 0) { + pt->nr--; + entry--; + remove_wait_queue(entry->wait_address,&entry->wait); + fput(entry->filp); + } while (p) { - struct poll_table_entry * entry; struct poll_table_page *old; entry = p->entry; @@ -73,33 +92,36 @@ void __pollwait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p) { - struct poll_table_page *table = p->table; - - if (!table || POLL_TABLE_FULL(table)) { - struct poll_table_page *new_table; + struct poll_table_entry * entry; - new_table = (struct poll_table_page *) __get_free_page(GFP_KERNEL); - if (!new_table) { - p->error = -ENOMEM; - __set_current_state(TASK_RUNNING); - return; + if(p->nr < POLL_TABLE_INTERNAL) { + entry = p->internal+p->nr++; + } else { + struct poll_table_page *table = p->table; + + if (!table || POLL_TABLE_FULL(table)) { + struct poll_table_page *new_table; + + new_table = kmalloc(PAGE_SIZE, GFP_KERNEL); + if (!new_table) { + p->error = -ENOMEM; + __set_current_state(TASK_RUNNING); + return; + } + new_table->entry = new_table->entries; + new_table->next = table; + p->table = new_table; + table = new_table; } - new_table->entry = new_table->entries; - new_table->next = table; - p->table = new_table; - table = new_table; - } - - /* Add a new entry */ - { - struct poll_table_entry * entry = table->entry; + entry = table->entry; table->entry = entry+1; - get_file(filp); - entry->filp = filp; - entry->wait_address = wait_address; - init_waitqueue_entry(&entry->wait, current); - add_wait_queue(wait_address,&entry->wait); } + /* Add a new entry */ + get_file(filp); + entry->filp = filp; + entry->wait_address = wait_address; + init_waitqueue_entry(&entry->wait, current); + add_wait_queue(wait_address,&entry->wait); } #define __IN(fds, n) (fds->in + n) @@ -233,14 +255,18 @@ return retval; } -static void *select_bits_alloc(int size) +#define SELECT_INLINE_BYTES 32 +static inline void *select_bits_alloc(int size, void* internal) { + if(size <= SELECT_INLINE_BYTES) + return internal; return kmalloc(6 * size, GFP_KERNEL); } -static void select_bits_free(void *bits, int size) +static inline void select_bits_free(void *bits, void* internal) { - kfree(bits); + if(bits != internal) + kfree(bits); } /* @@ -254,10 +280,12 @@ #define MAX_SELECT_SECONDS \ ((unsigned long) (MAX_SCHEDULE_TIMEOUT / HZ)-1) + asmlinkage long sys_select(int n, fd_set *inp, fd_set *outp, fd_set *exp, struct timeval *tvp) { fd_set_bits fds; + char ibuf[6*SELECT_INLINE_BYTES]; char *bits; long timeout; int ret, size; @@ -295,7 +323,7 @@ */ ret = -ENOMEM; size = FDS_BYTES(n); - bits = select_bits_alloc(size); + bits = select_bits_alloc(size, ibuf); if (!bits) goto out_nofds; fds.in = (unsigned long *) bits; @@ -340,12 +368,18 @@ set_fd_set(n, exp, fds.res_ex); out: - select_bits_free(bits, size); + select_bits_free(bits, ibuf); out_nofds: return ret; } -#define POLLFD_PER_PAGE ((PAGE_SIZE) / sizeof(struct pollfd)) +struct poll_list { + struct poll_list *next; + int len; + struct pollfd entries[0]; +}; + +#define POLLFD_PER_PAGE ((PAGE_SIZE-sizeof(struct poll_list)) / sizeof(struct pollfd)) static void do_pollfd(unsigned int num, struct pollfd * fdpage, poll_table ** pwait, int *count) @@ -379,39 +413,44 @@ } } -static int do_poll(unsigned int nfds, unsigned int nchunks, unsigned int nleft, - struct pollfd *fds[], poll_table *wait, long timeout) +static int do_poll(int nfds, struct poll_list *list, + poll_table *wait, long timeout) { - int count; + int count = 0; poll_table* pt = wait; - + for (;;) { - unsigned int i; - + struct poll_list* walk; set_current_state(TASK_INTERRUPTIBLE); - count = 0; - for (i=0; i < nchunks; i++) - do_pollfd(POLLFD_PER_PAGE, fds[i], &pt, &count); - if (nleft) - do_pollfd(nleft, fds[nchunks], &pt, &count); + walk = list; + while(walk != NULL) { + do_pollfd( walk->len, walk->entries, &pt, &count); + walk = walk->next; + } pt = NULL; if (count || !timeout || signal_pending(current)) break; count = wait->error; if (count) break; + timeout = schedule_timeout(timeout); } current->state = TASK_RUNNING; return count; } +#define INLINE_POLL_COUNT 24 asmlinkage long sys_poll(struct pollfd * ufds, unsigned int nfds, long timeout) { - int i, j, fdcount, err; - struct pollfd **fds; + int fdcount, err; + unsigned int i; + struct poll_list *pollwalk; + struct { + struct poll_list head; + struct pollfd entries[INLINE_POLL_COUNT]; + } polldata; poll_table table, *wait; - int nchunks, nleft; /* Do a sanity check on nfds ... */ if (nfds > current->files->max_fds) @@ -431,63 +470,65 @@ wait = NULL; err = -ENOMEM; - fds = NULL; - if (nfds != 0) { - fds = (struct pollfd **)kmalloc( - (1 + (nfds - 1) / POLLFD_PER_PAGE) * sizeof(struct pollfd *), - GFP_KERNEL); - if (fds == NULL) - goto out; - } + polldata.head.next = NULL; + polldata.head.len = INLINE_POLL_COUNT; + if(nfds <= INLINE_POLL_COUNT) + polldata.head.len = nfds; - nchunks = 0; - nleft = nfds; - while (nleft > POLLFD_PER_PAGE) { /* allocate complete PAGE_SIZE chunks */ - fds[nchunks] = (struct pollfd *)__get_free_page(GFP_KERNEL); - if (fds[nchunks] == NULL) + pollwalk = &polldata.head; + i = nfds; + err = -ENOMEM; + goto start; + while(i!=0) { + struct poll_list *pp; + pp = kmalloc(sizeof(struct poll_list)+ + sizeof(struct pollfd)* + (i>POLLFD_PER_PAGE?POLLFD_PER_PAGE:i), + GFP_KERNEL); + if(pp==NULL) goto out_fds; - nchunks++; - nleft -= POLLFD_PER_PAGE; - } - if (nleft) { /* allocate last PAGE_SIZE chunk, only nleft elements used */ - fds[nchunks] = (struct pollfd *)__get_free_page(GFP_KERNEL); - if (fds[nchunks] == NULL) + pp->next=NULL; + pp->len = (i>POLLFD_PER_PAGE?POLLFD_PER_PAGE:i); + pollwalk->next = pp; + pollwalk = pp; +start: + if (copy_from_user(pollwalk+1, ufds + nfds-i, + sizeof(struct pollfd)*pollwalk->len)) { + err = -EFAULT; goto out_fds; + } + i -= pollwalk->len; } + + fdcount = do_poll(nfds, &polldata.head, + wait, timeout); + /* OK, now copy the revents fields back to user space. */ + i = nfds; + pollwalk = &polldata.head; err = -EFAULT; - for (i=0; i < nchunks; i++) - if (copy_from_user(fds[i], ufds + i*POLLFD_PER_PAGE, PAGE_SIZE)) - goto out_fds1; - if (nleft) { - if (copy_from_user(fds[nchunks], ufds + nchunks*POLLFD_PER_PAGE, - nleft * sizeof(struct pollfd))) - goto out_fds1; + while(pollwalk != NULL) { + struct pollfd * fds = pollwalk->entries; + int j; + + for (j=0; j < pollwalk->len; j++, ufds++) { + if(__put_user(fds[j].revents, &ufds->revents)) + goto out_fds; + } + i -= pollwalk->len; + pollwalk = pollwalk->next; } - - fdcount = do_poll(nfds, nchunks, nleft, fds, wait, timeout); - - /* OK, now copy the revents fields back to user space. */ - for(i=0; i < nchunks; i++) - for (j=0; j < POLLFD_PER_PAGE; j++, ufds++) - __put_user((fds[i] + j)->revents, &ufds->revents); - if (nleft) - for (j=0; j < nleft; j++, ufds++) - __put_user((fds[nchunks] + j)->revents, &ufds->revents); - err = fdcount; if (!fdcount && signal_pending(current)) err = -EINTR; -out_fds1: - if (nleft) - free_page((unsigned long)(fds[nchunks])); out_fds: - for (i=0; i < nchunks; i++) - free_page((unsigned long)(fds[i])); - if (nfds != 0) - kfree(fds); -out: + pollwalk = polldata.head.next; + while(pollwalk!=NULL) { + struct poll_list *pp = pollwalk->next; + kfree(pollwalk); + pollwalk = pp; + } poll_freewait(&table); return err; } --- 2.4/include/linux/poll.h Thu Jan 4 23:51:10 2001 +++ build-2.4/include/linux/poll.h Fri Jan 5 13:46:30 2001 @@ -12,9 +12,18 @@ struct poll_table_page; +struct poll_table_entry { + struct file * filp; + wait_queue_t wait; + wait_queue_head_t * wait_address; +}; + +#define POLL_TABLE_INTERNAL 8 typedef struct poll_table_struct { int error; + int nr; struct poll_table_page * table; + struct poll_table_entry internal[POLL_TABLE_INTERNAL]; } poll_table; extern void __pollwait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p); @@ -28,6 +37,7 @@ static inline void poll_initwait(poll_table* pt) { pt->error = 0; + pt->nr = 0; pt->table = NULL; } extern void poll_freewait(poll_table* pt);
--------------B640E89F46A42E17A441425D--
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