| Message ID | 73dc827a12ef577ae7e644dcf34a5c0e443ab42f.1596012787.git.qemu_oss@crudebyte.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | 9pfs: readdir optimization | expand |
On Mittwoch, 29. Juli 2020 10:12:33 CEST Christian Schoenebeck wrote: > The newly added function v9fs_co_readdir_many() retrieves multiple > directory entries with a single fs driver request. It is intended to > replace uses of v9fs_co_readdir(), the latter only retrives a single > directory entry per fs driver request instead. > > The reason for this planned replacement is that for every fs driver > request the coroutine is dispatched from main I/O thread to a > background I/O thread and eventually dispatched back to main I/O > thread. Hopping between threads adds latency. So if a 9pfs Treaddir > request reads a large amount of directory entries, this currently > sums up to huge latencies of several hundred ms or even more. So > using v9fs_co_readdir_many() instead of v9fs_co_readdir() will > provide significant performance improvements. > > Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com> > --- > hw/9pfs/9p.h | 22 +++++++ > hw/9pfs/codir.c | 165 ++++++++++++++++++++++++++++++++++++++++++++++++ > hw/9pfs/coth.h | 3 + > 3 files changed, 190 insertions(+) > > diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h > index 561774e843..93b7030edf 100644 > --- a/hw/9pfs/9p.h > +++ b/hw/9pfs/9p.h > @@ -215,6 +215,28 @@ static inline void v9fs_readdir_init(V9fsDir *dir) > qemu_co_mutex_init(&dir->readdir_mutex); > } > > +/** > + * Type for 9p fs drivers' (a.k.a. 9p backends) result of readdir requests, > + * which is a chained list of directory entries. > + */ > +typedef struct V9fsDirEnt { > + /* mandatory (must not be NULL) information for all readdir requests */ > + struct dirent *dent; > + /* > + * optional (may be NULL): A full stat of each directory entry is just > + * done if explicitly told to fs driver. > + */ > + struct stat *st; > + /* > + * instead of an array, directory entries are always returned as > + * chained list, that's because the amount of entries retrieved by fs > + * drivers is dependent on the individual entries' name (since response > + * messages are size limited), so the final amount cannot be estimated > + * before hand > + */ > + struct V9fsDirEnt *next; > +} V9fsDirEnt; > + > /* > * Filled by fs driver on open and other > * calls. > diff --git a/hw/9pfs/codir.c b/hw/9pfs/codir.c > index ff57fb8619..8a8128ea13 100644 > --- a/hw/9pfs/codir.c > +++ b/hw/9pfs/codir.c > @@ -38,6 +38,10 @@ static int do_readdir(V9fsPDU *pdu, V9fsFidState *fidp, > struct dirent **dent) return err; > } > > +/* > + * TODO: This will be removed for performance reasons. > + * Use v9fs_co_readdir_many() instead. > + */ > int coroutine_fn v9fs_co_readdir(V9fsPDU *pdu, V9fsFidState *fidp, > struct dirent **dent) > { > @@ -52,6 +56,167 @@ int coroutine_fn v9fs_co_readdir(V9fsPDU *pdu, > V9fsFidState *fidp, return err; > } > > +/* > + * This is solely executed on a background IO thread. > + * > + * See v9fs_co_readdir_many() (as its only user) below for details. > + */ > +static int do_readdir_many(V9fsPDU *pdu, V9fsFidState *fidp, > + struct V9fsDirEnt **entries, off_t offset, > + int32_t maxsize, bool dostat) > +{ > + V9fsState *s = pdu->s; > + V9fsString name; > + int len, err = 0; > + int32_t size = 0; > + off_t saved_dir_pos; > + struct dirent *dent; > + struct V9fsDirEnt *e = NULL; > + V9fsPath path; > + struct stat stbuf; > + > + *entries = NULL; > + v9fs_path_init(&path); > + > + /* > + * TODO: Here should be a warn_report_once() if lock failed. > + * > + * With a good 9p client we should not get into concurrency here, > + * because a good client would not use the same fid for concurrent > + * requests. We do the lock here for safety reasons though. However > + * the client would then suffer performance issues, so better log that > + * issue here. > + */ > + v9fs_readdir_lock(&fidp->fs.dir); > + > + /* seek directory to requested initial position */ > + if (offset == 0) { > + s->ops->rewinddir(&s->ctx, &fidp->fs); > + } else { > + s->ops->seekdir(&s->ctx, &fidp->fs, offset); > + } > + > + /* save the directory position */ > + saved_dir_pos = s->ops->telldir(&s->ctx, &fidp->fs); > + if (saved_dir_pos < 0) { > + err = saved_dir_pos; > + goto out; > + } > + > + while (true) { To address your concern about aborting a readdir request, one could add here: if (v9fs_request_cancelled(pdu)) { err = -EINTR; break; } That's a minimal invasive change, so it would be Ok to add it to this patch already, or at any time later as a separate patch in future, as you like. > + /* get directory entry from fs driver */ > + err = do_readdir(pdu, fidp, &dent); > + if (err || !dent) { > + break; > + } > + > + /* > + * stop this loop as soon as it would exceed the allowed maximum > + * response message size for the directory entries collected so > far, + * because anything beyond that size would need to be > discarded by + * 9p controller (main thread / top half) anyway > + */ > + v9fs_string_init(&name); > + v9fs_string_sprintf(&name, "%s", dent->d_name); > + len = v9fs_readdir_response_size(&name); > + v9fs_string_free(&name); > + if (size + len > maxsize) { > + /* this is not an error case actually */ > + break; > + } > + > + /* append next node to result chain */ > + if (!e) { > + *entries = e = g_malloc0(sizeof(V9fsDirEnt)); > + } else { > + e = e->next = g_malloc0(sizeof(V9fsDirEnt)); > + } > + e->dent = g_malloc0(sizeof(struct dirent)); > + memcpy(e->dent, dent, sizeof(struct dirent)); > + > + /* perform a full stat() for directory entry if requested by caller > */ + if (dostat) { > + err = s->ops->name_to_path( > + &s->ctx, &fidp->path, dent->d_name, &path > + ); > + if (err < 0) { > + err = -errno; > + break; > + } > + > + err = s->ops->lstat(&s->ctx, &path, &stbuf); > + if (err < 0) { > + err = -errno; > + break; > + } > + > + e->st = g_malloc0(sizeof(struct stat)); > + memcpy(e->st, &stbuf, sizeof(struct stat)); > + } > + > + size += len; > + saved_dir_pos = dent->d_off; > + } > + > + /* restore (last) saved position */ > + s->ops->seekdir(&s->ctx, &fidp->fs, saved_dir_pos); > + > +out: > + v9fs_readdir_unlock(&fidp->fs.dir); > + v9fs_path_free(&path); > + if (err < 0) { > + return err; > + } > + return size; > +} > + > +/** > + * @brief Reads multiple directory entries in one rush. > + * > + * Retrieves the requested (max. amount of) directory entries from the fs > + * driver. This function must only be called by the main IO thread (top > half). + * Internally this function call will be dispatched to a background > IO thread + * (bottom half) where it is eventually executed by the fs > driver. + * > + * @discussion Acquiring multiple directory entries in one rush from the fs > + * driver, instead of retrieving each directory entry individually, is > very + * beneficial from performance point of view. Because for every fs > driver + * request latency is added, which in practice could lead to > overall + * latencies of several hundred ms for reading all entries (of > just a single + * directory) if every directory entry was individually > requested from fs + * driver. > + * > + * @note You must @b ALWAYS call @c v9fs_free_dirents(entries) after > calling + * v9fs_co_readdir_many(), both on success and on error cases of > this + * function, to avoid memory leaks once @p entries are no longer > needed. + * > + * @param pdu - the causing 9p (T_readdir) client request > + * @param fidp - already opened directory where readdir shall be performed > on + * @param entries - output for directory entries (must not be NULL) + * > @param offset - initial position inside the directory the function shall + > * seek to before retrieving the directory entries + * > @param maxsize - maximum result message body size (in bytes) > + * @param dostat - whether a stat() should be performed and returned for > + * each directory entry > + * @returns resulting response message body size (in bytes) on success, > + * negative error code otherwise > + */ > +int coroutine_fn v9fs_co_readdir_many(V9fsPDU *pdu, V9fsFidState *fidp, > + struct V9fsDirEnt **entries, > + off_t offset, int32_t maxsize, > + bool dostat) > +{ > + int err = 0; > + > + if (v9fs_request_cancelled(pdu)) { > + return -EINTR; > + } > + v9fs_co_run_in_worker({ > + err = do_readdir_many(pdu, fidp, entries, offset, maxsize, dostat); > + }); > + return err; > +} > + > off_t v9fs_co_telldir(V9fsPDU *pdu, V9fsFidState *fidp) > { > off_t err; > diff --git a/hw/9pfs/coth.h b/hw/9pfs/coth.h > index c2cdc7a9ea..fd4a45bc7c 100644 > --- a/hw/9pfs/coth.h > +++ b/hw/9pfs/coth.h > @@ -49,6 +49,9 @@ > void co_run_in_worker_bh(void *); > int coroutine_fn v9fs_co_readlink(V9fsPDU *, V9fsPath *, V9fsString *); > int coroutine_fn v9fs_co_readdir(V9fsPDU *, V9fsFidState *, struct dirent > **); +int coroutine_fn v9fs_co_readdir_many(V9fsPDU *, V9fsFidState *, > + struct V9fsDirEnt **, off_t, int32_t, > + bool); > off_t coroutine_fn v9fs_co_telldir(V9fsPDU *, V9fsFidState *); > void coroutine_fn v9fs_co_seekdir(V9fsPDU *, V9fsFidState *, off_t); > void coroutine_fn v9fs_co_rewinddir(V9fsPDU *, V9fsFidState *);
On Donnerstag, 30. Juli 2020 12:08:33 CEST Christian Schoenebeck wrote: > > @@ -52,6 +56,167 @@ int coroutine_fn v9fs_co_readdir(V9fsPDU *pdu, > > V9fsFidState *fidp, return err; > > > > } > > > > +/* > > + * This is solely executed on a background IO thread. > > + * > > + * See v9fs_co_readdir_many() (as its only user) below for details. > > + */ > > +static int do_readdir_many(V9fsPDU *pdu, V9fsFidState *fidp, > > + struct V9fsDirEnt **entries, off_t offset, > > + int32_t maxsize, bool dostat) > > +{ > > + V9fsState *s = pdu->s; > > + V9fsString name; > > + int len, err = 0; > > + int32_t size = 0; > > + off_t saved_dir_pos; > > + struct dirent *dent; > > + struct V9fsDirEnt *e = NULL; > > + V9fsPath path; > > + struct stat stbuf; > > + > > + *entries = NULL; > > + v9fs_path_init(&path); > > + > > + /* > > + * TODO: Here should be a warn_report_once() if lock failed. > > + * > > + * With a good 9p client we should not get into concurrency here, > > + * because a good client would not use the same fid for concurrent > > + * requests. We do the lock here for safety reasons though. However > > + * the client would then suffer performance issues, so better log > > that > > + * issue here. > > + */ > > + v9fs_readdir_lock(&fidp->fs.dir); > > + > > + /* seek directory to requested initial position */ > > + if (offset == 0) { > > + s->ops->rewinddir(&s->ctx, &fidp->fs); > > + } else { > > + s->ops->seekdir(&s->ctx, &fidp->fs, offset); > > + } > > + > > + /* save the directory position */ > > + saved_dir_pos = s->ops->telldir(&s->ctx, &fidp->fs); > > + if (saved_dir_pos < 0) { > > + err = saved_dir_pos; > > + goto out; > > + } > > + > > + while (true) { > > To address your concern about aborting a readdir request, one could add > here: > > if (v9fs_request_cancelled(pdu)) { > err = -EINTR; > break; > } > > That's a minimal invasive change, so it would be Ok to add it to this patch > already, or at any time later as a separate patch in future, as you like. I've applied this change now, as well as the minor comment change in patch 3 and pushed this overall patch set onto my queue: https://github.com/cschoenebeck/qemu/commits/master My plan is to send a PR once 5.1 is released (in presumably ~1-2 weeks), unless I hear back from you on anything else on this patch set. Best regards Christian Schoenebeck
diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h index 561774e843..93b7030edf 100644 --- a/hw/9pfs/9p.h +++ b/hw/9pfs/9p.h @@ -215,6 +215,28 @@ static inline void v9fs_readdir_init(V9fsDir *dir) qemu_co_mutex_init(&dir->readdir_mutex); } +/** + * Type for 9p fs drivers' (a.k.a. 9p backends) result of readdir requests, + * which is a chained list of directory entries. + */ +typedef struct V9fsDirEnt { + /* mandatory (must not be NULL) information for all readdir requests */ + struct dirent *dent; + /* + * optional (may be NULL): A full stat of each directory entry is just + * done if explicitly told to fs driver. + */ + struct stat *st; + /* + * instead of an array, directory entries are always returned as + * chained list, that's because the amount of entries retrieved by fs + * drivers is dependent on the individual entries' name (since response + * messages are size limited), so the final amount cannot be estimated + * before hand + */ + struct V9fsDirEnt *next; +} V9fsDirEnt; + /* * Filled by fs driver on open and other * calls. diff --git a/hw/9pfs/codir.c b/hw/9pfs/codir.c index ff57fb8619..8a8128ea13 100644 --- a/hw/9pfs/codir.c +++ b/hw/9pfs/codir.c @@ -38,6 +38,10 @@ static int do_readdir(V9fsPDU *pdu, V9fsFidState *fidp, struct dirent **dent) return err; } +/* + * TODO: This will be removed for performance reasons. + * Use v9fs_co_readdir_many() instead. + */ int coroutine_fn v9fs_co_readdir(V9fsPDU *pdu, V9fsFidState *fidp, struct dirent **dent) { @@ -52,6 +56,167 @@ int coroutine_fn v9fs_co_readdir(V9fsPDU *pdu, V9fsFidState *fidp, return err; } +/* + * This is solely executed on a background IO thread. + * + * See v9fs_co_readdir_many() (as its only user) below for details. + */ +static int do_readdir_many(V9fsPDU *pdu, V9fsFidState *fidp, + struct V9fsDirEnt **entries, off_t offset, + int32_t maxsize, bool dostat) +{ + V9fsState *s = pdu->s; + V9fsString name; + int len, err = 0; + int32_t size = 0; + off_t saved_dir_pos; + struct dirent *dent; + struct V9fsDirEnt *e = NULL; + V9fsPath path; + struct stat stbuf; + + *entries = NULL; + v9fs_path_init(&path); + + /* + * TODO: Here should be a warn_report_once() if lock failed. + * + * With a good 9p client we should not get into concurrency here, + * because a good client would not use the same fid for concurrent + * requests. We do the lock here for safety reasons though. However + * the client would then suffer performance issues, so better log that + * issue here. + */ + v9fs_readdir_lock(&fidp->fs.dir); + + /* seek directory to requested initial position */ + if (offset == 0) { + s->ops->rewinddir(&s->ctx, &fidp->fs); + } else { + s->ops->seekdir(&s->ctx, &fidp->fs, offset); + } + + /* save the directory position */ + saved_dir_pos = s->ops->telldir(&s->ctx, &fidp->fs); + if (saved_dir_pos < 0) { + err = saved_dir_pos; + goto out; + } + + while (true) { + /* get directory entry from fs driver */ + err = do_readdir(pdu, fidp, &dent); + if (err || !dent) { + break; + } + + /* + * stop this loop as soon as it would exceed the allowed maximum + * response message size for the directory entries collected so far, + * because anything beyond that size would need to be discarded by + * 9p controller (main thread / top half) anyway + */ + v9fs_string_init(&name); + v9fs_string_sprintf(&name, "%s", dent->d_name); + len = v9fs_readdir_response_size(&name); + v9fs_string_free(&name); + if (size + len > maxsize) { + /* this is not an error case actually */ + break; + } + + /* append next node to result chain */ + if (!e) { + *entries = e = g_malloc0(sizeof(V9fsDirEnt)); + } else { + e = e->next = g_malloc0(sizeof(V9fsDirEnt)); + } + e->dent = g_malloc0(sizeof(struct dirent)); + memcpy(e->dent, dent, sizeof(struct dirent)); + + /* perform a full stat() for directory entry if requested by caller */ + if (dostat) { + err = s->ops->name_to_path( + &s->ctx, &fidp->path, dent->d_name, &path + ); + if (err < 0) { + err = -errno; + break; + } + + err = s->ops->lstat(&s->ctx, &path, &stbuf); + if (err < 0) { + err = -errno; + break; + } + + e->st = g_malloc0(sizeof(struct stat)); + memcpy(e->st, &stbuf, sizeof(struct stat)); + } + + size += len; + saved_dir_pos = dent->d_off; + } + + /* restore (last) saved position */ + s->ops->seekdir(&s->ctx, &fidp->fs, saved_dir_pos); + +out: + v9fs_readdir_unlock(&fidp->fs.dir); + v9fs_path_free(&path); + if (err < 0) { + return err; + } + return size; +} + +/** + * @brief Reads multiple directory entries in one rush. + * + * Retrieves the requested (max. amount of) directory entries from the fs + * driver. This function must only be called by the main IO thread (top half). + * Internally this function call will be dispatched to a background IO thread + * (bottom half) where it is eventually executed by the fs driver. + * + * @discussion Acquiring multiple directory entries in one rush from the fs + * driver, instead of retrieving each directory entry individually, is very + * beneficial from performance point of view. Because for every fs driver + * request latency is added, which in practice could lead to overall + * latencies of several hundred ms for reading all entries (of just a single + * directory) if every directory entry was individually requested from fs + * driver. + * + * @note You must @b ALWAYS call @c v9fs_free_dirents(entries) after calling + * v9fs_co_readdir_many(), both on success and on error cases of this + * function, to avoid memory leaks once @p entries are no longer needed. + * + * @param pdu - the causing 9p (T_readdir) client request + * @param fidp - already opened directory where readdir shall be performed on + * @param entries - output for directory entries (must not be NULL) + * @param offset - initial position inside the directory the function shall + * seek to before retrieving the directory entries + * @param maxsize - maximum result message body size (in bytes) + * @param dostat - whether a stat() should be performed and returned for + * each directory entry + * @returns resulting response message body size (in bytes) on success, + * negative error code otherwise + */ +int coroutine_fn v9fs_co_readdir_many(V9fsPDU *pdu, V9fsFidState *fidp, + struct V9fsDirEnt **entries, + off_t offset, int32_t maxsize, + bool dostat) +{ + int err = 0; + + if (v9fs_request_cancelled(pdu)) { + return -EINTR; + } + v9fs_co_run_in_worker({ + err = do_readdir_many(pdu, fidp, entries, offset, maxsize, dostat); + }); + return err; +} + off_t v9fs_co_telldir(V9fsPDU *pdu, V9fsFidState *fidp) { off_t err; diff --git a/hw/9pfs/coth.h b/hw/9pfs/coth.h index c2cdc7a9ea..fd4a45bc7c 100644 --- a/hw/9pfs/coth.h +++ b/hw/9pfs/coth.h @@ -49,6 +49,9 @@ void co_run_in_worker_bh(void *); int coroutine_fn v9fs_co_readlink(V9fsPDU *, V9fsPath *, V9fsString *); int coroutine_fn v9fs_co_readdir(V9fsPDU *, V9fsFidState *, struct dirent **); +int coroutine_fn v9fs_co_readdir_many(V9fsPDU *, V9fsFidState *, + struct V9fsDirEnt **, off_t, int32_t, + bool); off_t coroutine_fn v9fs_co_telldir(V9fsPDU *, V9fsFidState *); void coroutine_fn v9fs_co_seekdir(V9fsPDU *, V9fsFidState *, off_t); void coroutine_fn v9fs_co_rewinddir(V9fsPDU *, V9fsFidState *);
The newly added function v9fs_co_readdir_many() retrieves multiple directory entries with a single fs driver request. It is intended to replace uses of v9fs_co_readdir(), the latter only retrives a single directory entry per fs driver request instead. The reason for this planned replacement is that for every fs driver request the coroutine is dispatched from main I/O thread to a background I/O thread and eventually dispatched back to main I/O thread. Hopping between threads adds latency. So if a 9pfs Treaddir request reads a large amount of directory entries, this currently sums up to huge latencies of several hundred ms or even more. So using v9fs_co_readdir_many() instead of v9fs_co_readdir() will provide significant performance improvements. Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com> --- hw/9pfs/9p.h | 22 +++++++ hw/9pfs/codir.c | 165 ++++++++++++++++++++++++++++++++++++++++++++++++ hw/9pfs/coth.h | 3 + 3 files changed, 190 insertions(+)