backend/local: Avoid polluting page cache when uploading local files to remote backends
This patch makes rclone keep linux page cache usage under control when
uploading local files to remote backends. When opening a file it issues
FADV_SEQUENTIAL to configure read ahead strategy. While reading
the file it issues FADV_DONTNEED every 128kB to free page cache from
already consumed pages.
```
fadvise64(5, 0, 0, POSIX_FADV_SEQUENTIAL) = 0
read(5, "\324\375\251\376\213\361\240\224>\t5E\301\331X\274^\203oA\353\303.2'\206z\177N\27fB"..., 32768) = 32768
read(5, "\361\311\vW!\354_\317hf\276t\307\30L\351\272T\342C\243\370\240\213\355\210\v\221\201\177[\333"..., 32768) = 32768
read(5, ":\371\337Gn\355C\322\334 \253f\373\277\301;\215\n\240\347\305\6N\257\313\4\365\276ANq!"..., 32768) = 32768
read(5, "\312\243\360P\263\242\267H\304\240Y\310\367sT\321\256\6[b\310\224\361\344$Ms\234\5\314\306i"..., 32768) = 32768
fadvise64(5, 0, 131072, POSIX_FADV_DONTNEED) = 0
read(5, "m\251\7a\306\226\366-\v~\"\216\353\342~0\fht\315DK0\236.\\\201!A#\177\320"..., 32768) = 32768
read(5, "\7\324\207,\205\360\376\307\276\254\250\232\21G\323n\255\354\234\257P\322y\3502\37\246\21\334^42"..., 32768) = 32768
read(5, "e{*\225\223R\320\212EG:^\302\377\242\337\10\222J\16A\305\0\353\354\326P\336\357A|-"..., 32768) = 32768
read(5, "n\23XA4*R\352\234\257\364\355Y\204t9T\363\33\357\333\3674\246\221T\360\226\326G\354\374"..., 32768) = 32768
fadvise64(5, 131072, 131072, POSIX_FADV_DONTNEED) = 0
read(5, "SX\331\251}\24\353\37\310#\307|h%\372\34\310\3070YX\250s\2269\242\236\371\302z\357_"..., 32768) = 32768
read(5, "\177\3500\236Y\245\376NIY\177\360p!\337L]\2726\206@\240\246pG\213\254N\274\226\303\357"..., 32768) = 32768
read(5, "\242$*\364\217U\264]\221Y\245\342r\t\253\25Hr\363\263\364\336\322\t\325\325\f\37z\324\201\351"..., 32768) = 32768
read(5, "\2305\242\366\370\203tM\226<\230\25\316(9\25x\2\376\212\346Q\223 \353\225\323\264jf|\216"..., 32768) = 32768
fadvise64(5, 262144, 131072, POSIX_FADV_DONTNEED) = 0
```
Page cache consumption per file can be checked with tools like [pcstat](https://github.com/tobert/pcstat).
This patch does not have a performance impact. Please find below results
of an experiment comparing local copy of 1GB file with and without this
patch.
With the patch:
```
(mmt/fadvise)$ pcstat 1GB.bin.1
+-----------+----------------+------------+-----------+---------+
| Name | Size (bytes) | Pages | Cached | Percent |
|-----------+----------------+------------+-----------+---------|
| 1GB.bin.1 | 1073741824 | 262144 | 0 | 000.000 |
+-----------+----------------+------------+-----------+---------+
(mmt/fadvise)$ taskset -c 0 /usr/bin/time -v ./rclone copy 1GB.bin.1 /var/empty/rclone
Command being timed: "./rclone copy 1GB.bin.1 /var/empty/rclone"
User time (seconds): 13.19
System time (seconds): 1.12
Percent of CPU this job got: 96%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:14.81
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 27660
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 0
Minor (reclaiming a frame) page faults: 2212
Voluntary context switches: 5755
Involuntary context switches: 9782
Swaps: 0
File system inputs: 4155264
File system outputs: 2097152
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0
(mmt/fadvise)$ pcstat 1GB.bin.1
+-----------+----------------+------------+-----------+---------+
| Name | Size (bytes) | Pages | Cached | Percent |
|-----------+----------------+------------+-----------+---------|
| 1GB.bin.1 | 1073741824 | 262144 | 0 | 000.000 |
+-----------+----------------+------------+-----------+---------+
```
Without the patch:
```
(master)$ taskset -c 0 /usr/bin/time -v ./rclone copy 1GB.bin.1 /var/empty/rclone
Command being timed: "./rclone copy 1GB.bin.1 /var/empty/rclone"
User time (seconds): 14.46
System time (seconds): 0.81
Percent of CPU this job got: 93%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:16.41
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 27600
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 0
Minor (reclaiming a frame) page faults: 2228
Voluntary context switches: 7190
Involuntary context switches: 1980
Swaps: 0
File system inputs: 2097152
File system outputs: 2097152
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0
(master)$ pcstat 1GB.bin.1
+-----------+----------------+------------+-----------+---------+
| Name | Size (bytes) | Pages | Cached | Percent |
|-----------+----------------+------------+-----------+---------|
| 1GB.bin.1 | 1073741824 | 262144 | 262144 | 100.000 |
+-----------+----------------+------------+-----------+---------+
```
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