On 12/15/2017 12:59 AM, Andreas Dilger wrote:

On Dec 14, 2017, at 3:00 PM, Krzysztof Opasiak [off-list ref] wrote:

quoted

Add rlimit-events calls to file descriptors management
code to allow tracing of FD usage.

This allows userspace process (monitor) to get notification when
other process (subject) uses given amount of file descriptors.

This can be used to for example asynchronously monitor number
of open FD's in system services instead of polling with
predefined interval.

I'm not involved in this area of code, but one optimization question:

quoted

+static unsigned int count_open_fds(struct fdtable *fdt)
+{
+	unsigned int maxfd = fdt->max_fds;
+	unsigned int maxbit = maxfd / BITS_PER_LONG;
+	unsigned int count = 0;
+	int i;
+
+	i = find_next_zero_bit(fdt->full_fds_bits, maxbit, 0);
+	/* If there is no free fds */
+	if (i > maxbit)
+		return maxfd;
+#if BITS_PER_LONG == 32
+#define HWEIGHT_LONG hweight32
+#else
+#define HWEIGHT_LONG hweight64
+#endif
+
+	count += i * BITS_PER_LONG;
+	for (; i < maxbit; ++i)
+		count += HWEIGHT_LONG(fdt->open_fds[i]);
+
+#undef HWEIGHT_LONG
+	return count;
+}

Since find_next_zero_bit() needs to process all of the words anyway
as well as lots of extra operations that add overhead, it looks more
efficient to just compute HWEIGHT_LONG(open_fds[]) for the whole array.

I may try to measure this but I'm not sure now because there are two 
levels of bit maps. First one (open_fds) is a real bitmap of open file 
descriptors (which can be quite large) while full_fds_bits is a second 
level bitmap (which is 8 times smaller) and if bit there is set it means 
that whole byte in lower bitmap (open_fds) is set. Because we are always 
opening the lowest fd number that is available we should have quite 
large contiguous region in the beginning so using this 
find_next_zero_bit() at first should allow us to skip this region.

But anyway I can try your suggestion and check if it speeds up.

Best regards,
-- 
Krzysztof Opasiak
Samsung R&D Institute Poland
Samsung Electronics