Bump efivarfs_get_variable throughput to 50 variables/second from 33⅓ #259
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…rate limit) for each EFI variable read
Nominally this rate limit is defined to avoid... getting rate-limited?
But it already severely limits the rate to unusable
‒ on two of my real systems this makes efibootmgr take 168ms/194ms,
which accounts for 95%/82% of the run-time
(and this is under strace, so it's 100% of the run-time) ‒
for klapki 0.2, this accounts for 36% and a large (140ms!)
start-up delay, and for klapki 0.3 it's well over 50%.
Well before you'd ever run afoul of the real limit.
Discounting "20ms" as "The user is not going to notice." is baffling.
efibootmgr is infuriatingly slow. 20ms is ping-to-america level.
Worse yet, the entire kernel rate-limiter amounts to fs/efivarfs/file.c
-- >8 --
static ssize_t efivarfs_file_read(struct file *file, char __user *userbuf,
size_t count, loff_t *ppos)
{
struct efivar_entry *var = file->private_data;
unsigned long datasize = 0;
u32 attributes;
void *data;
ssize_t size = 0;
int err;
while (!__ratelimit(&file->f_cred->user->ratelimit))
msleep(50);
-- >8 --
this is the alloc_uid() ratelimit with 1s interval + 100 burst.
This means that we can (best-case) read 50 variables
(read(...), read(0)) instantly, then do so again the next second.
Best-case because the current implementation is broken anyway:
it sleeps for 10ms after the attribute read (sure),
and then for 10ms after the /two/ reads to read the rest of the
variable (bad).
This limits libefivar to 33⅓ variables per second.
Most systems have roughly this many variables. Most programs only
care about a very thin subset of them, and scarcely come close to
reading enough to run afoul of the kernel limit. But even if they
did, this limit is /significantly harsher/ than the kernel limit ‒
it doesn't increase it (how could it? the limit's already there!),
but severely increases latency for /every single read/, instead of
just those over the rate.
It's strictly worse than just not doing it.
This was confirmed experimentally with strace -TTTT /bin/wc * * * * *
(note the many every-variable expansions so it's noticeable):
there is both visually a very obvious "big burst, little slowdown"
oscillation, but also (non-efivarfs reads filtered out)
$ awk '/^read/ {print $NF}' ss | tr -d '<>' | sort -n | cut -c -6 | uniq -c | sort -n
1 0.8998
1 0.9015
1 0.9581
1 0.9585
1 0.9586
5 0.0013
9 0.0005
9 0.0012
46 0.0011
70 0.0010
84 0.0008
85 0.0009
102 0.0006
115 0.0007
or indeed
$ awk '/^read/ {print $NF}' ss | tr -d '<>' | sort -n | cut -c -5 | uniq -c | sort -n
1 0.899
1 0.901
3 0.958
130 0.001
395 0.000
(130+395)/2=262½ variables read in under a millisecond,
and 4½ got limited.
But, much more importantly, the first screenful was free:
99% of programs that don't read every variable
over and over and over, but fit well within the 33
(klapki's 7 and efibootmgr's 8,
this is with the firmware's base boot entries + two additional ones;
there isn't a non-hypothetical system in existence with 25 more boot entries).
Fixes: https://bugs.debian.org/1056344
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Read the whole variable at once, then copy out the attributes (for a description of efivarfs rate limiting, see rhboot#258) Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
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Read the whole variable at once, then copy out the attributes (for a description of efivarfs rate limiting, see #258).
Draft because based on #258, otherwise g2g.