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littlefs/bd/lfs3_kiwibd.c
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Christopher Haster d3dd927de3 runners: emubd/kiwibd: Adopted emulated simtime API 
This is based on some work in external benchmarks. What's worked well
there is emulating a global simtime based on per-byte estimates.

This moves the emulated simtime into emubd/kiwibd, and extends the idea
with both per-byte and per-op timing estimates for hopefully more
realistic results.

---

The problem is how NAND flash reads work.

Per-byte timing estimates are surprisingly accurate for NOR flash. There
is some overhead for sending the address, but it's mostly dominated by
bus cost (~20ns/B [1]).

NAND flash, on the otherhand, technically does support byte-level reads,
but first needs to read into 2KiB buffer. Surprisingly, these are pretty
close in cost (~19ns/B bus [2] vs ~12ns/B buffer [2]).

This close-ness makes modeling NAND flash difficult. If we set
read_size=1, we risk hiding the cost of small reads, which littlefs3 is
full of (rbyd lookups). If we set read_size=2048, we unfairly penalize
littlefs3 for the same reason.

---

The solution here is to expose both per-byte and per-op timing
estimates. This lets you model NAND reads using two data points:

  ^
  |                                realtime --> ...............o
  |                                             :    .....'''' :
  |                              ...............:''''  ^       :
  |                              :....'''''            |       :
  |               ..........::::::                  simtime    :
  |          .....:''''                                        :
  |o....:::::.....:                                            :
  |:                                                           :
  |:                                                           :
  +:-----------------------------------------------------------:>
   min read                                              max read

Where:

  bus_timing = 19ns
  buffer_timing = 25us
  buffer_size = 2KiB
  erase_size = 128KiB

  min_read = buffer_timing
  max_read = (erase_size/buffer_size)*buffer_timing - buffer_timing
  read_timing = min_read
  readed_timing = ((max_read - min_read)/erase_size) + bus_timing

  simtime = reads*read_timing + readed*readed_timing
            (per-op)            (per-byte)

This should correctly penalize small reads without complicating
emubd/kiwibd too much.

That's the idea anyways! It will take some use to understand if this is
a reasonable approach.

As a plus, this is a superset of the per-byte model, so both can be used
for realistic vs idealistic simulations (and to test the bus+buffer
model itself).

1: https://www.winbond.com/resource-files/W25Q256JV%20SPI%20RevQ%2002072025%20Plus.pdf
2: https://www.winbond.com/resource-files/W25N01GV%20Rev%20R%20070323.pdf
2026-02-10 15:28:32 -06:00

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/*
* kiwibd - A lightweight variant of emubd, useful for emulating large
* disks backed by a file or in RAM.
*
* Unlike emubd, file-backed disks are _not_ mirrored in RAM. kiwibd has
* fewer features than emubd, prioritizing speed for benchmarking.
*
*
*/
#ifndef _POSIX_C_SOURCE
#define _POSIX_C_SOURCE 199309L
#endif
#include "bd/lfs3_kiwibd.h"
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <time.h>
// low-level flash memory emulation
// read data
static inline void lfs3_kiwibd_memread(const struct lfs3_cfg *cfg,
void *restrict dst, const void *restrict src, size_t size) {
(void)cfg;
memcpy(dst, src, size);
}
static inline void lfs3_kiwibd_memprog(const struct lfs3_cfg *cfg,
void *restrict dst, const void *restrict src, size_t size) {
lfs3_kiwibd_t *bd = cfg->context;
// emulating nor-masking?
if (bd->cfg->erase_value == -2) {
uint8_t *dst_ = dst;
const uint8_t *src_ = src;
for (size_t i = 0; i < size; i++) {
dst_[i] &= src_[i];
}
} else {
memcpy(dst, src, size);
}
}
static inline void lfs3_kiwibd_memerase(const struct lfs3_cfg *cfg,
void *restrict dst, size_t size) {
lfs3_kiwibd_t *bd = cfg->context;
// emulating erase value?
if (bd->cfg->erase_value != -1) {
memset(dst,
(bd->cfg->erase_value >= 0)
? bd->cfg->erase_value
: 0xff,
size);
}
}
// this is slightly different from lfs3_kiwibd_memerase in that we use
// lfs3_kiwibd_memzero when we need to unconditionally zero memory
static inline void lfs3_kiwibd_memzero(const struct lfs3_cfg *cfg,
void *restrict dst, size_t size) {
lfs3_kiwibd_t *bd = cfg->context;
memset(dst,
(bd->cfg->erase_value == -1) ? 0
: (bd->cfg->erase_value >= 0) ? bd->cfg->erase_value
: (bd->cfg->erase_value == -2) ? 0xff
: 0,
size);
}
// kiwibd create/destroy
int lfs3_kiwibd_createcfg(const struct lfs3_cfg *cfg, const char *path,
const struct lfs3_kiwibd_cfg *bdcfg) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_createcfg("
"%p {"
".context=%p, "
".read=%p, "
".prog=%p, "
".erase=%p, "
".sync=%p, "
".read_size=%"PRIu32", "
".prog_size=%"PRIu32", "
".block_size=%"PRIu32", "
".block_count=%"PRIu32"}, "
"\"%s\", "
"%p {"
".erase_value=%"PRId32", "
".buffer=%p, "
".read_sleep=%"PRIu64", "
".prog_sleep=%"PRIu64", "
".erase_sleep=%"PRIu64"})",
(void*)cfg,
cfg->context,
(void*)(uintptr_t)cfg->read,
(void*)(uintptr_t)cfg->prog,
(void*)(uintptr_t)cfg->erase,
(void*)(uintptr_t)cfg->sync,
cfg->read_size,
cfg->prog_size,
cfg->block_size,
cfg->block_count,
path,
(void*)bdcfg,
bdcfg->erase_value,
bdcfg->buffer,
bdcfg->read_sleep,
bdcfg->prog_sleep,
bdcfg->erase_sleep);
lfs3_kiwibd_t *bd = cfg->context;
bd->cfg = bdcfg;
// setup some initial state
bd->reads = 0;
bd->progs = 0;
bd->erases = 0;
bd->readed = 0;
bd->progged = 0;
bd->erased = 0;
bd->fd = -1;
if (path) {
bd->u.scratch = NULL;
} else {
bd->u.mem = NULL;
}
int err;
// if we have a path, try to open the backing file
if (path) {
bd->fd = open(path, O_RDWR | O_CREAT, 0666);
if (bd->fd < 0) {
err = -errno;
goto failed;
}
// allocate a scratch buffer to help with zeroing/masking/etc
bd->u.scratch = malloc(cfg->block_size);
if (!bd->u.scratch) {
err = LFS3_ERR_NOMEM;
goto failed;
}
// zero for reproducibility
lfs3_kiwibd_memzero(cfg, bd->u.scratch, cfg->block_size);
for (lfs3_block_t i = 0; i < cfg->block_count; i++) {
ssize_t res = write(bd->fd,
bd->u.scratch,
cfg->block_size);
if (res < 0) {
err = -errno;
goto failed;
}
}
// otherwise, try to malloc a big memory array
} else {
bd->u.mem = malloc((size_t)cfg->block_size * cfg->block_count);
if (!bd->u.mem) {
err = LFS3_ERR_NOMEM;
goto failed;
}
// zero for reproducibility
lfs3_kiwibd_memzero(cfg, bd->u.mem,
(size_t)cfg->block_size * cfg->block_count);
}
LFS3_KIWIBD_TRACE("lfs3_kiwibd_createcfg -> %d", 0);
return 0;
failed:;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_createcfg -> %d", err);
// clean up memory
if (bd->fd >= 0) {
close(bd->fd);
free(bd->u.scratch);
} else {
free(bd->u.mem);
}
return err;
}
int lfs3_kiwibd_create(const struct lfs3_cfg *cfg, const char *path) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_create("
"%p {"
".context=%p, "
".read=%p, "
".prog=%p, "
".erase=%p, "
".sync=%p, "
".read_size=%"PRIu32", "
".prog_size=%"PRIu32", "
".block_size=%"PRIu32", "
".block_count=%"PRIu32"}, "
"\"%s\")",
(void*)cfg,
cfg->context,
(void*)(uintptr_t)cfg->read,
(void*)(uintptr_t)cfg->prog,
(void*)(uintptr_t)cfg->erase,
(void*)(uintptr_t)cfg->sync,
cfg->read_size,
cfg->prog_size,
cfg->block_size,
cfg->block_count,
path);
static const struct lfs3_kiwibd_cfg defaults = {.erase_value=-1};
int err = lfs3_kiwibd_createcfg(cfg, path, &defaults);
LFS3_KIWIBD_TRACE("lfs3_kiwibd_create -> %d", err);
return err;
}
int lfs3_kiwibd_destroy(const struct lfs3_cfg *cfg) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_destroy(%p)", (void*)cfg);
lfs3_kiwibd_t *bd = cfg->context;
// clean up memory
if (bd->fd >= 0) {
close(bd->fd);
free(bd->u.scratch);
} else {
free(bd->u.mem);
}
LFS3_KIWIBD_TRACE("lfs3_kiwibd_destroy -> %d", 0);
return 0;
}
// block device API
int lfs3_kiwibd_read(const struct lfs3_cfg *cfg, lfs3_block_t block,
lfs3_off_t off, void *buffer, lfs3_size_t size) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_read(%p, "
"0x%"PRIx32", %"PRIu32", %p, %"PRIu32")",
(void*)cfg, block, off, buffer, size);
lfs3_kiwibd_t *bd = cfg->context;
// check if read is valid
LFS3_ASSERT(block < cfg->block_count);
LFS3_ASSERT(off % cfg->read_size == 0);
LFS3_ASSERT(size % cfg->read_size == 0);
LFS3_ASSERT(off+size <= cfg->block_size);
// read in file?
if (bd->fd >= 0) {
lfs3_kiwibd_memerase(cfg,
bd->u.scratch,
cfg->block_size);
off_t res = lseek(bd->fd,
(off_t)block*cfg->block_size + (off_t)off,
SEEK_SET);
if (res < 0) {
int err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_read -> %d", err);
return err;
}
ssize_t res_ = read(bd->fd, buffer, size);
if (res_ < 0) {
int err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_read -> %d", err);
return err;
}
// read in RAM?
} else {
lfs3_kiwibd_memread(cfg,
buffer,
&bd->u.mem[(size_t)block*cfg->block_size + (size_t)off],
size);
}
// track reads
bd->reads += 1;
bd->readed += size;
if (bd->cfg->read_sleep) {
int err = nanosleep(&(struct timespec){
.tv_sec=bd->cfg->read_sleep/1000000000,
.tv_nsec=bd->cfg->read_sleep%1000000000},
NULL);
if (err) {
err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_read -> %d", err);
return err;
}
}
LFS3_KIWIBD_TRACE("lfs3_kiwibd_read -> %d", 0);
return 0;
}
int lfs3_kiwibd_prog(const struct lfs3_cfg *cfg, lfs3_block_t block,
lfs3_off_t off, const void *buffer, lfs3_size_t size) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_prog(%p, "
"0x%"PRIx32", %"PRIu32", %p, %"PRIu32")",
(void*)cfg, block, off, buffer, size);
lfs3_kiwibd_t *bd = cfg->context;
// check if write is valid
LFS3_ASSERT(block < cfg->block_count);
LFS3_ASSERT(off % cfg->prog_size == 0);
LFS3_ASSERT(size % cfg->prog_size == 0);
LFS3_ASSERT(off+size <= cfg->block_size);
// prog in file?
if (bd->fd >= 0) {
// were we erased properly?
if (bd->cfg->erase_value >= 0) {
off_t res = lseek(bd->fd,
(off_t)block*cfg->block_size + (off_t)off,
SEEK_SET);
if (res < 0) {
int err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_prog -> %d", err);
return err;
}
ssize_t res_ = read(bd->fd, bd->u.scratch, size);
if (res_ < 0) {
int err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_prog -> %d", err);
return err;
}
for (lfs3_off_t i = 0; i < size; i++) {
LFS3_ASSERT(bd->u.scratch[i] == bd->cfg->erase_value);
}
}
// masking progs?
if (bd->cfg->erase_value == -2) {
off_t res = lseek(bd->fd,
(off_t)block*cfg->block_size + (off_t)off,
SEEK_SET);
if (res < 0) {
int err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_prog -> %d", err);
return err;
}
ssize_t res_ = read(bd->fd, bd->u.scratch, size);
if (res_ < 0) {
int err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_prog -> %d", err);
return err;
}
lfs3_kiwibd_memprog(cfg, bd->u.scratch, buffer, size);
res = lseek(bd->fd,
(off_t)block*cfg->block_size + (off_t)off,
SEEK_SET);
if (res < 0) {
int err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_prog -> %d", err);
return err;
}
res_ = write(bd->fd, bd->u.scratch, size);
if (res_ < 0) {
int err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_prog -> %d", err);
return err;
}
// normal progs?
} else {
off_t res = lseek(bd->fd,
(off_t)block*cfg->block_size + (off_t)off,
SEEK_SET);
if (res < 0) {
int err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_prog -> %d", err);
return err;
}
ssize_t res_ = write(bd->fd, buffer, size);
if (res_ < 0) {
int err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_prog -> %d", err);
return err;
}
}
// prog in RAM?
} else {
// were we erased properly?
if (bd->cfg->erase_value >= 0) {
for (lfs3_off_t i = 0; i < size; i++) {
LFS3_ASSERT(
bd->u.mem[(size_t)block*cfg->block_size + (size_t)off]
== bd->cfg->erase_value);
}
}
lfs3_kiwibd_memprog(cfg,
&bd->u.mem[(size_t)block*cfg->block_size + (size_t)off],
buffer,
size);
}
// track progs
bd->progs += 1;
bd->progged += size;
if (bd->cfg->prog_sleep) {
int err = nanosleep(&(struct timespec){
.tv_sec=bd->cfg->prog_sleep/1000000000,
.tv_nsec=bd->cfg->prog_sleep%1000000000},
NULL);
if (err) {
err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_prog -> %d", err);
return err;
}
}
LFS3_KIWIBD_TRACE("lfs3_kiwibd_prog -> %d", 0);
return 0;
}
int lfs3_kiwibd_erase(const struct lfs3_cfg *cfg, lfs3_block_t block) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_erase(%p, 0x%"PRIx32" (%"PRIu32"))",
(void*)cfg, block, cfg->block_size);
lfs3_kiwibd_t *bd = cfg->context;
// check if erase is valid
LFS3_ASSERT(block < cfg->block_count);
// emulate an erase value?
if (bd->cfg->erase_value != -1) {
// erase in file?
if (bd->fd >= 0) {
off_t res = lseek(bd->fd,
(off_t)block*cfg->block_size,
SEEK_SET);
if (res < 0) {
int err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_erase -> %d", err);
return err;
}
lfs3_kiwibd_memerase(cfg,
bd->u.scratch,
cfg->block_size);
ssize_t res_ = write(bd->fd,
bd->u.scratch,
cfg->block_size);
if (res_ < 0) {
int err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_erase -> %d", err);
return err;
}
// erase in RAM?
} else {
lfs3_kiwibd_memerase(cfg,
&bd->u.mem[(size_t)block*cfg->block_size],
cfg->block_size);
}
}
erased:;
// track erases
bd->erases += 1;
bd->erased += cfg->block_size;
if (bd->cfg->erase_sleep) {
int err = nanosleep(&(struct timespec){
.tv_sec=bd->cfg->erase_sleep/1000000000,
.tv_nsec=bd->cfg->erase_sleep%1000000000},
NULL);
if (err) {
err = -errno;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_erase -> %d", err);
return err;
}
}
LFS3_KIWIBD_TRACE("lfs3_kiwibd_erase -> %d", 0);
return 0;
}
int lfs3_kiwibd_sync(const struct lfs3_cfg *cfg) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_sync(%p)", (void*)cfg);
// in theory we could actually sync here, but if our goal is
// performance, why bother?
//
// filebd may be a better block device is your goal is actual
// storage
// sync is a noop
(void)cfg;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_sync -> %d", 0);
return 0;
}
/// Additional kiwibd features ///
lfs3_kiwibd_sns_t lfs3_kiwibd_simtime(const struct lfs3_cfg *cfg) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_simtime(%p)", (void*)cfg);
lfs3_kiwibd_t *bd = cfg->context;
// error if all possible timings are zero
if (bd->cfg->reads_timing == 0
&& bd->cfg->progs_timing == 0
&& bd->cfg->erases_timing == 0
&& bd->cfg->readed_timing == 0
&& bd->cfg->progged_timing == 0
&& bd->cfg->erased_timing == 0) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_simtime -> %d", LFS3_ERR_NOTSUP);
return LFS3_ERR_NOTSUP;
}
lfs3_kiwibd_ns_t ns
= (bd->cfg->reads_timing * bd->reads)
+ (bd->cfg->progs_timing * bd->progs)
+ (bd->cfg->erases_timing * bd->erases)
+ (bd->cfg->readed_timing * bd->readed)
+ (bd->cfg->progged_timing * bd->progged)
+ (bd->cfg->erased_timing * bd->erased);
LFS3_KIWIBD_TRACE("lfs3_kiwibd_simtime -> %"PRIu64, ns);
return ns;
}
int lfs3_kiwibd_simreset(const struct lfs3_cfg *cfg) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_simreset(%p)", (void*)cfg);
lfs3_kiwibd_t *bd = cfg->context;
bd->reads = 0;
bd->progs = 0;
bd->erases = 0;
bd->readed = 0;
bd->progged = 0;
bd->erased = 0;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_simreset -> %d", 0);
return 0;
}
lfs3_kiwibd_sio_t lfs3_kiwibd_reads(const struct lfs3_cfg *cfg) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_reads(%p)", (void*)cfg);
lfs3_kiwibd_t *bd = cfg->context;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_reads -> %"PRIu64, bd->reads);
return bd->reads;
}
lfs3_kiwibd_sio_t lfs3_kiwibd_progs(const struct lfs3_cfg *cfg) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_progs(%p)", (void*)cfg);
lfs3_kiwibd_t *bd = cfg->context;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_progs -> %"PRIu64, bd->progs);
return bd->progs;
}
lfs3_kiwibd_sio_t lfs3_kiwibd_erases(const struct lfs3_cfg *cfg) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_erases(%p)", (void*)cfg);
lfs3_kiwibd_t *bd = cfg->context;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_erases -> %"PRIu64, bd->erases);
return bd->erases;
}
lfs3_kiwibd_sio_t lfs3_kiwibd_readed(const struct lfs3_cfg *cfg) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_readed(%p)", (void*)cfg);
lfs3_kiwibd_t *bd = cfg->context;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_readed -> %"PRIu64, bd->readed);
return bd->readed;
}
lfs3_kiwibd_sio_t lfs3_kiwibd_progged(const struct lfs3_cfg *cfg) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_progged(%p)", (void*)cfg);
lfs3_kiwibd_t *bd = cfg->context;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_progged -> %"PRIu64, bd->progged);
return bd->progged;
}
lfs3_kiwibd_sio_t lfs3_kiwibd_erased(const struct lfs3_cfg *cfg) {
LFS3_KIWIBD_TRACE("lfs3_kiwibd_erased(%p)", (void*)cfg);
lfs3_kiwibd_t *bd = cfg->context;
LFS3_KIWIBD_TRACE("lfs3_kiwibd_erased -> %"PRIu64, bd->erased);
return bd->erased;
}
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